[java-spanner] No way to provide `transaction_tag` with Partitioned DML update

**Is your feature request related to a problem? Please describe.**
Not sure if that's a bug or a feature request. There is no way to assign a _transaction_tag_ (not request tag!) to a partitioned DML transaction.

**Describe the solution you'd like**
Well, give us a way to optionally pass a transaction tag.

**Additional context**
We'd like to use `transaction_tag`-s to make improvements to our CDC mechanism which utilizes Change Streams. Unlike request tags, transaction tags make it all the way to change stream's data change records and we'd like to use that. Unfortunately, we can't do it when we do a massive updates using Partitioned DML.