[Java] Header parameters (in: header) are accepted as method parameters in the generated google-api-client Java client but are never added to the HTTP request.

[artnan]

Summary

Fixes #22457

Header parameters (in: header) are accepted as method parameters in the generated
google-api-client Java client but are never added to the HTTP request. This PR is targeting java so mentioning the Java committee here @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08)

Changes

Updated api.mustache to add header parameters to HttpRequest before calling execute():

• Main ForHttpResponse method
• InputStream variant method
• Params map variant method

Testing

Generated a client with header parameters and verified headers are now properly sent locally.

PR checklist

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