feat(gax): add mutable extension accessors to RequestOptions

[olavloite]

Add get_extension_mut and get_extension_or_default_mut to the RequestOptionsExt trait and implement them for RequestOptions.

These methods allow client library layers to mutate request option extensions in place (such as injecting or updating headers in an existing HeaderMap) without needing to clone or re-insert the extension.

[gemini-code-assist]

Code Review

This pull request bumps the version of google-cloud-gax to 1.14.0 and adds get_extension_mut and get_extension_or_default_mut methods to RequestOptions to support mutable extension retrieval. The review feedback correctly points out that the Clone trait bound on get_extension_or_default_mut is unnecessary because the default value is inserted directly by value, and removing it would make the API more flexible.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.07%. Comparing base (0dac6ea) to head (01be876).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6254   +/-   ##
=======================================
  Coverage   96.07%   96.07%           
=======================================
  Files         275      275           
  Lines       68829    68860   +31     
=======================================
+ Hits        66126    66160   +34     
+ Misses       2703     2700    -3     

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[coryan]

I think you intended this as an example:

These methods allow client library layers to mutate request option extensions in place (such as injecting or updating headers in an existing HeaderMap) without needing to clone or re-insert the extension.

I must ask, if this is the only use-case you had in mind, would the changes in #6260 satisfy your needs? I would like to avoid two different ways to set headers.

If you had other cases in mind, then this looks good.