feat(generator/rust): prefer gaxi over gclient

[coryan]

This is one of the steps to create google-cloud-gax-internal, using
the contents from google-cloud-http-client.

Fixes #1543

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.19%. Comparing base (a9c0c0c) to head (5ef5000).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1546   +/-   ##
=======================================
  Coverage   95.19%   95.19%           
=======================================
  Files          39       39           
  Lines        1603     1603           
=======================================
  Hits         1526     1526           
  Misses         77       77           

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