feat(spanner): [WIP] support data client

[olavloite]

[WORK IN PROGRESS]

Add a data-plane client for Spanner.

Depends on googleapis/librarian#4072

[coryan]

/gcbrun

[coryan]

Thanks for the PR! I think you can skip the grpc-mock subdirectory in the first PR and things will be easier to review and merge.

Also filed googleapis/librarian#4073 because the grpc-mock thing is tedious. Do not worry, I believe we should continue to make progress with this PR (maybe split in smaller PRs) and leave the sidekick automation for later.

[coryan]

/gcbrun

[codecov]

Codecov Report

❌ Patch coverage is 0% with 403 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.25%. Comparing base (095167f) to head (eebfef2).
⚠️ Report is 44 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spanner/src/value.rs	0.00%	217 Missing ⚠️
src/spanner/src/read_write_transaction.rs	0.00%	43 Missing ⚠️
src/spanner/src/statement.rs	0.00%	31 Missing ⚠️
src/spanner/src/row.rs	0.00%	25 Missing ⚠️
src/gax-internal/src/grpc.rs	0.00%	21 Missing ⚠️
src/spanner/src/read_context.rs	0.00%	19 Missing ⚠️
src/spanner/src/client/builder.rs	0.00%	18 Missing ⚠️
src/spanner/src/partitioned_dml.rs	0.00%	10 Missing ⚠️
src/spanner/grpc-mock/src/lib.rs	0.00%	9 Missing ⚠️
src/spanner/src/client.rs	0.00%	9 Missing ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4701      +/-   ##
==========================================
- Coverage   95.05%   90.25%   -4.80%     
==========================================
  Files         198      208      +10     
  Lines        7581     7984     +403     
==========================================
  Hits         7206     7206              
- Misses        375      778     +403     

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