cleanup(spanner): generate grpc-mock

[dbolduc]

Fixes #5396

Replaces src/spanner/grpc-mock with a sidekick-generated version.

The return type for streams was changed from our other services. I went through and manually updated the existing tests.

The PR is split into multiple commits to maybe make reviewing easier.

@olavloite - You have many pending PRs. If any of them are using the Box::pin(...) return type, we need to coordinate merging them before/after this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.74%. Comparing base (5d354cb) to head (99fdf81).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5433      +/-   ##
==========================================
- Coverage   97.74%   97.74%   -0.01%     
==========================================
  Files         216      216              
  Lines       47592    47549      -43     
==========================================
- Hits        46519    46476      -43     
  Misses       1073     1073              

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