Support transaction finishing automatically with 'idle timeout'

The goal is to have a mode for finishing transactions automatically once no new spans are added to it for N milliseconds, or a general timeout.

We need to define the spec in the develop.sentry.dev docs and align with other implementations of it. This is an approach used by the JS on the browser (and React Native).

Note On Android: we introduced an opt-out for the auto activity transaction from closing on rendering (it should only add/end span instead of the whole transaction). In that case might not use the idle timeout transaction when that flag is opt-in and rely on the user programatic finish call instead.

[marandaneto]

with the waitForChildren feat, io.sentry.traces.activity.auto-finish.enable will likely get deprecated, I don’t see a reason why having it anymore. https://docs.sentry.io/platforms/android/performance/instrumentation/automatic-instrumentation/

[rhcarvalho]

We may also consider how this special transaction will behave as we focus more on spans, and one day be able to ingest data without buffering full transactions in memory.

Perhaps, if we make the functionality of “automatically finish” available to any span, that would put us in a better future position, not tied to transactions.