chore: share ownership with Spanner team

[dbolduc]

Share ownership of Spanner code with the Spanner team. They should be able to make progress on development without having to wait on the cloud-sdk-rust-team.

cc: @olavloite

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.77%. Comparing base (4832962) to head (fb3cdd5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5341      +/-   ##
==========================================
- Coverage   97.78%   97.77%   -0.01%     
==========================================
  Files         222      222              
  Lines       46113    46113              
==========================================
- Hits        45090    45088       -2     
- Misses       1023     1025       +2     

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