[java-firestore] Firestore TransactionManager and explicit commit/rollback

Hello again,

Another aspect of Spring Data for Firestore we're making is `@Transactional` annotation support. Basically the user annotates a method with that annotation and every operation inside the method is supposed to be done inside a single transaction.

Right now we see there is only the option of running a given anonymous function inside a transaction and logically this can work as we need it to. However, Spring Framework comes with a ton of boilerplate and support around this that essentially assumes the old-fashioned transaction manager / explicit commit/ rollback is available like from old-fashioned SQL databases.

Are there plans to add this explicit transactionmanager and control to Firestore?

We actually had this exact same situation with the Spanner client lib, but fortunately a few months after we got started the transactionmanager was added. Are there such plans for Firestore?

teammates: @meltsufin @elefeint @dmitry-s @dzou