feat: Last statement sample

[sgorse123]

Support of the last statement option was added in #3647 by @olavloite.

This change adds a sample of how to use the option in the last DML statement of a transaction.

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/src/main/java/com/example/spanner/LastStatementSample.java:45, tag spanner_last_statement
• samples/snippets/src/main/java/com/example/spanner/PgLastStatementSample.java:45, tag spanner_postgresql_last_statement

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