Add support for custom payloads to client-swarm

[ballard26]

The intended use-case for this change is enabling client-swarm to produce messages that are properly formatted for a given schema on a given Iceberg topic. After this PR is merged a follow-up PR will be created enabling Iceberg topics in the large messages scale test.

Backports Required

• [x] none - not a bug fix
• [ ] none - this is a backport
• [ ] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v25.3.x
• [ ] v25.2.x
• [ ] v25.1.x

Release Notes

• none

[vbotbuildovich]

CI test results

test results on build#77621

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
AioLimitTest	test_aio_limit	null	integration	https://buildkite.com/redpanda/redpanda/builds/77621#019b0582-d063-42d3-a82e-19052a95b616	FAIL	0/1		https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=AioLimitTest&test_method=test_aio_limit
PartitionReassignmentsTest	test_reassignments_cancel	null	integration	https://buildkite.com/redpanda/redpanda/builds/77621#019b059a-ac63-49b0-bd85-ab8565830cf1	FLAKY	17/21	Test PASSES after retries.No significant increase in flaky rate(baseline=0.1165, p0=0.4173, reject_threshold=0.0100. adj_baseline=0.3104, p1=0.0902, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=PartitionReassignmentsTest&test_method=test_reassignments_cancel

test results on build#77843

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
EndToEndCloudTopicsTest	test_delete_records	null	integration	https://buildkite.com/redpanda/redpanda/builds/77843#019b1fb5-9ac7-4e9b-ae44-978014f553fa	FLAKY	30/31	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0097, p0=1.0000, reject_threshold=0.0100. adj_baseline=0.0289, p1=0.4147, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=EndToEndCloudTopicsTest&test_method=test_delete_records
PartitionReassignmentsTest	test_reassignments_cancel	null	integration	https://buildkite.com/redpanda/redpanda/builds/77843#019b1fbb-1d9f-4bbe-8d50-4b4f71172a14	FLAKY	9/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0815, p0=0.5726, reject_threshold=0.0100. adj_baseline=0.2251, p1=0.3049, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=PartitionReassignmentsTest&test_method=test_reassignments_cancel

test results on build#77911

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
ShadowLinkingReplicationTests	test_replication_with_failures	null	integration	https://buildkite.com/redpanda/redpanda/builds/77911#019b247a-ce4d-47c8-a3c4-0de682f39f27	FLAKY	50/51	Test PASSES after retries.Inconclusive result after max retries(baseline=0.0025, p0=1.0000, reject_threshold=0.0100. adj_baseline=0.0075, p1=0.6850, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=ShadowLinkingReplicationTests&test_method=test_replication_with_failures
CrashLoopChecksTest	test_cloud_storage_client_misconfiguration	null	integration	https://buildkite.com/redpanda/redpanda/builds/77911#019b2473-d24c-4b1c-9267-bd9be233ce58	FAIL	0/51	Test FAILS after retries.Significant increase in flaky rate(baseline=0.9020, p0=0.0057, reject_threshold=0.0100)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=CrashLoopChecksTest&test_method=test_cloud_storage_client_misconfiguration
CrashLoopChecksTest	test_cloud_storage_client_misconfiguration	null	integration	https://buildkite.com/redpanda/redpanda/builds/77911#019b247a-ce4b-4d5a-9c5b-bed2ebd88fd1	FLAKY	3/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.9085, p0=0.9906, reject_threshold=0.0100. adj_baseline=0.9992, p1=0.0000, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=CrashLoopChecksTest&test_method=test_cloud_storage_client_misconfiguration
PartitionReassignmentsTest	test_reassignments_cancel	null	integration	https://buildkite.com/redpanda/redpanda/builds/77911#019b2473-d25a-451e-bbeb-1d68ee5023ba	FLAKY	10/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0463, p0=1.0000, reject_threshold=0.0100. adj_baseline=0.1326, p1=0.2412, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=PartitionReassignmentsTest&test_method=test_reassignments_cancel
PartitionReassignmentsTest	test_reassignments_cancel	null	integration	https://buildkite.com/redpanda/redpanda/builds/77911#019b247a-ce52-4fd1-a0d9-4a0a42e2d24f	FLAKY	18/21	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0824, p0=0.4993, reject_threshold=0.0100. adj_baseline=0.2274, p1=0.1341, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=PartitionReassignmentsTest&test_method=test_reassignments_cancel
RedpandaNodeOperationsSmokeTest	test_node_ops_smoke_test	{"cloud_storage_type": 1, "mixed_versions": false}	integration	https://buildkite.com/redpanda/redpanda/builds/77911#019b245e-32cd-43d0-a676-3d21ea0f4ad9	FAIL	0/1		https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=RedpandaNodeOperationsSmokeTest&test_method=test_node_ops_smoke_test

[ballard26]

Tests are failing due to https://github.com/redpanda-data/redpanda/pull/29003

[vbotbuildovich]

Retry command for Build#77911

please wait until all jobs are finished before running the slash command

/ci-repeat 1
skip-redpanda-build
skip-units
skip-rebase
tests/rptest/tests/crash_loop_checks_test.py::CrashLoopChecksTest.test_cloud_storage_client_misconfiguration