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 #78

Checklist

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

[codecov[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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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