perf(spanner): inline BeginTransaction with first query (step 1)

[olavloite]

Adds support for inlining the BeginTransaction option with the first query in a read-only transaction. This saves one round-trip to Spanner for multi-use read-only transactions.

This implementation is intentionally simple:

1. It does not support parallel queries at the start of the transaction.
2. It does not include error handling for the first query.
3. It only supports read-only transactions.

This is step 1. Follow-up pull requests addresses the above points.

A preliminary view of the full implementation after all steps can be seen in #5307

See also go/spanner-rust-inline-begin-transaction

[codecov]

Codecov Report

❌ Patch coverage is 98.03922% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.80%. Comparing base (e373f94) to head (d8af76e).

Files with missing lines	Patch %	Lines
src/spanner/src/read_only_transaction.rs	97.73%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5320      +/-   ##
==========================================
+ Coverage   97.79%   97.80%   +0.01%     
==========================================
  Files         220      220              
  Lines       45845    46066     +221     
==========================================
+ Hits        44832    45053     +221     
  Misses       1013     1013              

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