Feat: Configure idle transaction duration

[denrase]

📜 Description

Provide the autoFinishAfter duration in the constructor of SentryNavigatorObserver.

💡 Motivation and Context

Closes #667

💚 How did you test it?

Updated unit tests.

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[codecov-commenter]

Codecov Report

Merging #705 (a5dd161) into main (11b55fb) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #705   +/-   ##
=======================================
  Coverage   90.61%   90.61%           
=======================================
  Files         103      103           
  Lines        3259     3259           
=======================================
  Hits         2953     2953           
  Misses        306      306           

Impacted Files	Coverage Δ
.../lib/src/navigation/sentry_navigator_observer.dart	98.48% <100.00%> (ø)

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[marandaneto]

@denrase we could add this field to the docs too https://docs.sentry.io/platforms/flutter/performance/instrumentation/automatic-instrumentation/#routing-instumentation