cleanup: refactor headers in credentials

[chrischiedo]

This PR is an effort to resolve issue #1686.

While working on this PR, I considered two approaches:

1. Creating a utility function that takes the necessary parameters and returns the required headers
2. Adding a default headers implementation to the CredentialsTrait trait that has the common logic that can then be shared across the credentials

Option 1 was more straight forward (and easier to implement). Option 2, on the other hand, is a bit more involving (requiring more changes to existing code). Therefore, for this PR, I went with Option 1.

Note: I have added a utils module, following the structure recommended in the Rust Book here. Currently we have only the headers_util utility function, but more utility functions could be added when/if needed.

[coryan]

Thanks for the PR! I started most builds and we left some comments. @harkamaljot or @sai-sunder-s may have further comments.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.79%. Comparing base (4079324) to head (4358516).
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1741      +/-   ##
==========================================
- Coverage   95.83%   95.79%   -0.04%     
==========================================
  Files          49       50       +1     
  Lines        1919     1904      -15     
==========================================
- Hits         1839     1824      -15     
  Misses         80       80              

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[chrischiedo]

@sai-sunder-s: Here's the updated PR. Lmk if this addresses your feedback/comment.

cc/ @coryan

[sai-sunder-s]

Thank you for the change!

[coryan]

/gcbrun