new: StandardDatabase.fetch_transaction (#329)

[Moortiii]

Resolves #329.

Introduces a new method fetch_transaction on StandardDatabase that allows users to create a TransactionDatabase based on an existing Transaction, instead of starting a new transaction.

[apetenchea]

Thanks for the prompt followup!
Looks good overall and the logic is correct (also docs example is nice). The only thing which should be addressed is in executor.py, but that's merely a simplification.

[apetenchea]

Change looks good to me!

Unfortunately the docs CI is complaining of duplicate keys. Just use some different names in the example.

[Moortiii]

I see that there are some failing checks still, but they don't seem to be related to anything I've touched or something that should have been impacted by my changes.

[apetenchea]

The failing docs test is due to the fact that Pregel has been deprecated recently. That's an update we'll have to make on our end.

Thank you for your contribution!