sample(spanner): Add samples for dml returning

[diptanshumittal]

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 6 region tags.

• spanner/spanner_delete_dml_returning.rb:15, tag spanner_delete_dml_returning
• spanner/spanner_insert_dml_returning.rb:15, tag spanner_insert_dml_returning
• spanner/spanner_postgresql_delete_dml_returning.rb:15, tag spanner_postgresql_delete_dml_returning
• spanner/spanner_postgresql_insert_dml_returning.rb:15, tag spanner_postgresql_insert_dml_returning
• spanner/spanner_postgresql_update_dml_returning.rb:15, tag spanner_postgresql_update_dml_returning
• spanner/spanner_update_dml_returning.rb:15, tag spanner_update_dml_returning

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[NivedhaSenthil]

Please create separate files for each sample.