feat: propagate HTTP headers to HttpError type

[karel-rehor]

Closes #357

Proposed Changes

Briefly describe your proposed changes:

• Update HttpError to include a headers type
• In transport implementations, add headers from HTTP response to HttpError when an error is encountered.
• Provide an example showing how to work with retry-after header when encountering HTTP 429 or 503
• Update existing tests
• Add new appropriate tests

Checklist

• [x] CHANGELOG.md updated
• [x] Rebased/mergeable
• [x] A test has been added if appropriate
• [x] Tests pass
• [x] Commit messages are conventional
• [x] Sign CLA (if not already signed)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.57%. Comparing base (5cad6a9) to head (1ca7b22). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #369 +/- ## ======================================= Coverage 97.57% 97.57% ======================================= Files 23 23 Lines 907 908 +1 Branches 214 214 ======================================= + Hits 885 886 +1 Misses 10 10 Partials 12 12 ```

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