chore(spanner): directly execute queries and wait for first result

[olavloite]

This changes the way that queries are executed on Spanner from a model that uses a background task to read from the query stream, to a model that directly calls ExecuteStreamingSql when execute_query is called, and then waits for the first PartialResultSet or error to be received. This ensures that execute_query really executes the query and returns any errors that happens during query execution.

Closes #5685
Fixes #5673

[gemini-code-assist]

Code Review

This pull request refactors the ResultSet implementation to handle metadata initialization more robustly, replacing the background worker pattern with an explicit init_stream method. It also introduces ExplicitBeginParams to streamline transaction initialization and adds rustls as a dependency for secure connections. The reviewer suggests explicitly handling the Result from the rustls provider installation and refactoring redundant metadata logic in ResultSet to improve maintainability.

[codecov]

Codecov Report

❌ Patch coverage is 96.22642% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.88%. Comparing base (23f3c97) to head (e45381f).

Files with missing lines	Patch %	Lines
src/spanner/src/result_set.rs	95.26%	8 Missing ⚠️
src/spanner/src/read_write_transaction.rs	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5698      +/-   ##
==========================================
- Coverage   97.89%   97.88%   -0.02%     
==========================================
  Files         226      226              
  Lines       55536    55552      +16     
==========================================
+ Hits        54368    54378      +10     
- Misses       1168     1174       +6     

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