BE: Messages: Allow searching in message headers

[yeikel]

What changes did you make? (Give an overview)

Closes #687

Is there anything you'd like reviewers to focus on?

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• Unit checks

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

Check out Contributing and Code of Conduct

A picture of a cute animal (not mandatory but encouraged)

[yeikel]

@Haarolean Thank you for reviewing it

I believe that the E2E test failures are unrelated?

Is there anything else pending to merge this?

[Haarolean]

@yeikel yeah, there's one flaky test. Thanks for another contribution!

[yeikel]

@yeikel yeah, there's one flaky test. Thanks for another contribution!

Thank you for merging it :)