Spanner supports the ON CONFLICT clause for GoogleSQL INSERT statements to handle unique constraint

## Feature Feature Spanner supports the `ON CONFLICT` clause for GoogleSQL `INSERT` statements to handle unique constraint violations. For more information, see [ON CONFLICT DO NOTHING](https://cloud.google.com/spanner/docs/reference/standard-sql/dml-syntax#insert-on-conflict-do-nothing)and [ON CONFLICT DO UPDATE](https://cloud.google.com/spanner/docs/reference/standard-sql/dml-syntax#insert-on-conflict-do-update).