Add General purpose Write operation to the cross-language Spanner transform

[https://github.com/apache/beam/pull/12611](https://github.com/apache/beam/pull/12611) largely addresses this by adding individual xlang wrappers for delete/replace/insert/update/insert_or_update operations restricted to a single table.

The only thing missing is a general purpose "write" transform that can accept general mutations (multiple tables, multiple operations). To get that right will require some design work and likely some new features in portable schemas (unions).

Imported from Jira [BEAM-11287](https://issues.apache.org/jira/browse/BEAM-11287). Original Jira may contain additional context.
Reported by: piotr-szuberski.