feat(spanner): tests and samples for DML RETURNING

[devbww]

Add tests and samples for GOOGLE_STANDARD_SQL "THEN RETURN" and POSTGRESQL "RETURNING".

Note that Client::ExecuteQuery() can now execute a DML statement with a returning clause.

Previously we added support for RowStream::RowsModified() (#10102), which is now used here.

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This change is 

[google-cloud-cpp-bot]

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For commit: f5fe8b067563334789d8b5956d48c1952934682c

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[codecov]

Codecov Report

Base: 93.97% // Head: 93.84% // Decreases project coverage by -0.12% ⚠️

Coverage data is based on head (1caaa0e) compared to base (cc21bb0).
Patch coverage: 3.04% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10233      +/-   ##
==========================================
- Coverage   93.97%   93.84%   -0.13%     
==========================================
  Files        1540     1540              
  Lines      141901   142098     +197     
==========================================
+ Hits       133345   133353       +8     
- Misses       8556     8745     +189     

Impacted Files	Coverage Δ
google/cloud/spanner/client.h	100.00% <ø> (ø)
google/cloud/spanner/samples/postgresql_samples.cc	0.65% <0.00%> (-0.07%)	⬇️
google/cloud/spanner/samples/samples.cc	62.24% <1.85%> (-1.29%)	⬇️
...r/integration_tests/data_types_integration_test.cc	59.66% <5.74%> (-14.13%)	⬇️
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.65% <0.00%> (ø)
google/cloud/completion_queue_test.cc	97.32% <0.00%> (+0.19%)	⬆️
google/cloud/pubsub/subscriber_connection_test.cc	98.36% <0.00%> (+0.54%)	⬆️
...integration_tests/schema_admin_integration_test.cc	100.00% <0.00%> (+1.11%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 1caaa0e04e69ea3bf6f8090a141828604eaaf155

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[snippet-bot]

Here is the summary of changes.

You are about to add 6 region tags.

• google/cloud/spanner/samples/postgresql_samples.cc:222, tag spanner_postgresql_update_dml_returning
• google/cloud/spanner/samples/postgresql_samples.cc:254, tag spanner_postgresql_insert_dml_returning
• google/cloud/spanner/samples/postgresql_samples.cc:282, tag spanner_postgresql_delete_dml_returning
• google/cloud/spanner/samples/samples.cc:3281, tag spanner_update_dml_returning
• google/cloud/spanner/samples/samples.cc:3313, tag spanner_insert_dml_returning
• google/cloud/spanner/samples/samples.cc:3341, tag spanner_delete_dml_returning

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