feat: add support for custom query/gRPC headers

[alespour]

Proposed Changes

• Adds custom headers (from either ClientConfig and/or provided to query via WithHeader options) to a gRPC query request.
• Query call examples are now in "godoc" style - compilable (but not executed during go test) code in example_query_test.go, from which documentation examples are generated and linked with functions and types on pkg.go.dev portal, see fix: use godoc examples instead of code comments #78

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• Tests pass
• Commit messages are conventional

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.38%. Comparing base (f0bf1b6) to head (6615c39).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   83.09%   83.38%   +0.28%     
==========================================
  Files          12       12              
  Lines         994     1011      +17     
==========================================
+ Hits          826      843      +17     
  Misses        138      138              
  Partials       30       30              

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[karel-rehor]

The functional options pattern took a moment to understand, but it's neat. Tests pass locally. Everything else looks OK.