[CORE-2841] Transforms: Start consuming from an arbitrary offset (numeric from start/end or timestamp)

[oleiman]

This PR wires up the ability to configure the start offset of a transform at deploy time. This can be either a unix timestamp (ms since epoch) or an offset delta (+oo from start offset of -oo from end).

Includes rpk experience.

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
• [ ] v24.1.x
• [ ] v23.3.x
• [ ] v23.2.x

Release Notes

Improvements

• Adds the ability to start transform processing from an arbitrary offset on the input topic.

[oleiman]

/ci-repeat 1

[oleiman]

force push contents:

• introduce kafka::offset_delta
• reject offset "-0" in rpk
• minor CR changes

[oleiman]

force push empty diff (signoff)

[oleiman]

force push rebase dev to avoid conflicts on feature flag

[oleiman]

force push feature flag for the new position types, minor CR changes

[oleiman]

empty diff for commit signoff

[oleiman]

force push contents:

• serde shenanigans
• adjust feature gate to guard new offset alternatives only

[oleiman]

force push make legacy_transform_offset_options struct write-only.

[oleiman]

force push a dt test case

[oleiman]

force push to address some CR comments on the rpk side

[oleiman]

CI Failure is https://github.com/redpanda-data/redpanda/issues/20574 (known, unrelated)