github: Add config for Gemini code assist

[arjan-bal]

Add configuration to enable Gemini code reviews.

Source: https://github.com/grpc/grpc/blob/master/.gemini/config.yaml

RELEASE NOTES: N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.17%. Comparing base (3e475d7) to head (436eb76).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8873      +/-   ##
==========================================
+ Coverage   82.47%   83.17%   +0.70%     
==========================================
  Files         414      414              
  Lines       32751    32751              
==========================================
+ Hits        27011    27241     +230     
- Misses       4089     4091       +2     
+ Partials     1651     1419     -232     

see 31 files with indirect coverage changes

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[dfawley]

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