dynamic_modules: adds support for header removal

[mathetake]

Commit Message: dynamic_modules: adds support for header removal
Additional Description:

This allows dynamic modules to remove headers/trailers by allowing setter callbacks to accept null pointer values.

Risk Level: low
Testing: done
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

[repokitteh-read-only]

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🐱

Caused by: #38432 was opened by mathetake.

see: more, trace.

[mathetake]

/assign @mattklein123