feat: custom headers can be specified for queries/writes

[bednar]

Proposed Changes

Custom HTTP/gRPC headers can be specified globally by config or per request.

Checklist

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.70%. Comparing base (d148532) to head (27f4e5b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   95.59%   95.70%   +0.11%     
==========================================
  Files          12       12              
  Lines        1044     1072      +28     
  Branches      130      136       +6     
==========================================
+ Hits          998     1026      +28     
  Misses         11       11              
  Partials       35       35              

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[karel-rehor]

Tests past locally for me with dotnet 8.0.202. Everything makes sense.