feat: add tests for DML with Returning clause

[rajatbhatta]

This PR adds tests for running DML with Returning clause using the JDBC driver, and incorporates the following:

• Integration tests for running DML statements with Returning clause using PreparedStatement.
• Unit tests for running DML statements with Returning clause using JdbcStatement, for each of the available JDBC APIs execute, executeUpdate, executeQuery, executeBatchUpdate.
• The JDBC driver does not require any code changes for supporting DML with Returning clause, as all the required changes will be made in the Connection API (Connection API changes are being tracked at feat: support DML with Returning clause in Connection API java-spanner#1978).

[rajatbhatta]

Note: Integration tests are not expected to pass until DML with Returning clause support is launched.