Datastore: support for previous transaction id in transaction manager

[pcostell]

Is your feature request related to a problem? Please describe.
When performing transactions in Firestore in Datastore Mode, occasionally the commit will fail. The base client library retry runners will retry that transaction with the previous transaction id, which makes following commits more likely to succeed, however that support does not exist in Spring.

Describe the solution you'd like
DatastoreTransactionManager automatically handles setting the previous transaction id when run with EnableRetry.

Describe alternatives you've considered
Workarounds currently include using DatastoreTemplate.performTransaction, which leverages a retry runner which properly sets previous transaction id. This workaround however does not allow usage of the Transactional annotation.

[shivammm21]

Hi @pcostell, thanks for raising this issue.

I’d like to work on adding support for reusing the previous transaction ID in
DatastoreTransactionManager when retries are enabled. I’ll align the behavior
with DatastoreTemplate.performTransaction and add appropriate tests.

Please let me know if you have any suggestions or preferences before I start.
Thanks!