feat: Add snippets for Spanner DML with returning clause

[c2nes]

Samples are provided for INSERT, DELETE, and UPDATE in both GoogleSQL and PostgreSQL dialects. To provide a more compelling example for the INSERT case, a generated column has been added in the "create_database" example so that the generated value can be returned in the INSERT examples.

[snippet-bot]

Here is the summary of changes.

You are about to add 6 region tags.

• samples/samples/pg_snippets.py:546, tag spanner_postgresql_dml_insert_returning
• samples/samples/pg_snippets.py:891, tag spanner_postgresql_dml_update_returning
• samples/samples/pg_snippets.py:943, tag spanner_postgresql_dml_delete_returning
• samples/samples/snippets.py:1352, tag spanner_dml_update_returning
• samples/samples/snippets.py:1404, tag spanner_dml_delete_returning
• samples/samples/snippets.py:1540, tag spanner_dml_insert_returning

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[xinjiacs]

Thanks Chris, the samples look good to me!

There may be some testing configuration errors about the examples that need to be fixed.

[rajatbhatta]

LGTM.

[c2nes]

@rajatbhatta could you merge this for me when you have a chance? I don't think I have access to do so myself

[ansh0l]

Looks like some presubmits are still running after last push, we will take a look later again

[rajatbhatta]

@c2nes: I’ve enabled auto-merge on the PR. PR should automatically get merged once the kokoro tests pass.

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