Expose auto transaction idle time via options

[denrase]

• Provide the duration via sentry options.

Discussion: #643 (comment)

Relates to #643

[marandaneto]

Let's check if the JS SDK has it and if not, maybe we don't need it, we can always add it later, but it's always harder to remove it.

[marandaneto]

Exists on RN https://github.com/getsentry/sentry-react-native/blob/main/src/js/tracing/reactnativetracing.ts#L37-L53

[denrase]

@marandaneto I had the same issue when trying to introduce debug logs: How could we best provide access to SentryOptions in the SentryNavigatorObserver? We only have access to the Hub, which holds a private reference to the options.

[marandaneto]

@denrase what if SentryNavigatorObserver takes an optional param autoFinishAfter: Duration = Duration(seconds: 3)?

[denrase]

Yeah, would work if we just keep it locally and not involve the options at all.