Allow providers to override ambient transaction access

[roji]

Accessing Transaction.Current (in HandleAmbientTransactions) is a heavy operation - profiling shows it to take around 0.3% of the total running time.

Npgsql has connection string parameter to disable this check at the ADO level. Allowing providers to override HandleAmbientTransactions would allow EFCore.PG to skip this if the parameter is set.

HandleAmbientTransactions isn't a great name for a publicly-exposed method, but I don't really have anything better...

Related Npgsql PRs: npgsql/efcore.pg#1742, npgsql/npgsql#3570

[AndriySvyryd]

It would be better to just wrap Transaction.Current and expose it on ITransactionEnlistmentManager, this way all places using it would benefit.

[roji]

Good point for doing it for other places (e.g. BatchExecutor).

Though the intent here was to also allow skipping the _ambientTransactions checks that come after (Count, TryPeek), i.e. to skip HandleAmbientTransactions altogether (even if perf-wise it's probably not very significant). Should we do both? Am OK to also only do Transaction.Current.

[roji]

Note slight behavior change where the connection doesn't implement ITransactionEnlistmentManager but Transaction.Current is non-null. New behavior seems more correct.

[AndriySvyryd]

Though the intent here was to also allow skipping the _ambientTransactions checks that come after (Count, TryPeek), i.e. to skip HandleAmbientTransactions altogether (even if perf-wise it's probably not very significant). Should we do both? Am OK to also only do Transaction.Current.

If we expose HandleAmbientTransactions then we would also need to expose _ambientTransactions in some way in case the provider wants to do something else than skipping. I'd prefer to defer this for now until there's a another use case for this.

[roji]

Makes sense, changed HandleAmbientTransactions back to private.

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