in_http: parse msgpack payloads emitted by out_http.

[pwhelan]

Summary

Accept the msgpack payload that the out_http plugin emits. This is related to #8493, both are meant to solve an error that occurs when out_http is sent packets to in_http.

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Testing Before we can approve your change; please submit the following in a comment:

• [ ] Example configuration file for the change
• [ ] Debug log output from testing the change

• [ ] Attached Valgrind output that shows no leaks or memory corruption was found

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• [ ] Run local packaging test showing all targets (including any new ones) build.
• [ ] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [ ] Documentation required for this feature

Backporting

• [ ] Backport to latest stable release.

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