google-cloud-go spanner/v1.12.0

Features

• spanner: add metadata to RowIterator (#3050) (9a2289c), closes #1805
• spanner: export ToSpannerError (#3133) (b951d8b), closes #3122
• spanner: support rw-transaction with options (#3058) (5130694)
• spanner/spannertest: make SELECT list aliases visible to ORDER BY (#3054) (7d2d83e), closes #3043