feat(header-finalizer): support multiple headers with same name

[aslafy-z]

Related issue(s)

Closes #2206

Checklist

• I agree to follow this project's Code of Conduct.
• I have read, and I am following this repository's Contributing Guidelines.
• I have read the Security Policy.
• I have referenced an issue describing the bug/feature request.
• I have added tests that prove the correctness of my implementation.
• I have updated the documentation.

Description

This PR introduces support for multiple headers with the same name by allowing newline-separated values in the header finalizer. The values are split and added as individual headers, ensuring compatibility with applications that rely on this behavior.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.45%. Comparing base (de735f4) to head (899695c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2244      +/-   ##
==========================================
- Coverage   90.51%   90.45%   -0.07%     
==========================================
  Files         279      279              
  Lines       12246    12251       +5     
==========================================
- Hits        11085    11082       -3     
- Misses        916      921       +5     
- Partials      245      248       +3     

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[dadrus]

Thank you @aslafy-z very much for the PR. As you can see above, I added some comments related to styling.

[dadrus]

I'll merge it as soon as the 0.15.8 is released and merged into the main branch. Otherwise this feature will be affected by #2237.

[aslafy-z]

@dadrus Thanks for taking a look! I think the styling comments might not have been submitted. Could you double-check?

[aslafy-z]

@dadrus I applied the requested changes and rebased on main.