Fix: Use correct data/extra type in tracer

[denrase]

📜 Description

Fixes #679

💡 Motivation and Context

• The internal property _extra in the SentryTracer is exposed via data property and both are of type Map< string, string>
• The latter is used when intializing SentryTransactions extra property, which is of type Map<string, dynamic>
• By changing to Map<string, dynamic> we support the correct type and more complex hierarchies can be serialized further downstream.
• Serializing complex objects should work as long as every call of jsonEncode is provided jsonSerializationFallback implementation, which calls toString() on non-null objects.

💚 How did you test it?

• Added unit test.
• Checked that all calls of jsonEncode are provided with jsonSerializationFallback.

📝 Checklist

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

[codecov-commenter]

Codecov Report

Merging #693 (a3eb8a4) into main (1f32c29) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #693   +/-   ##
=======================================
  Coverage   90.65%   90.65%           
=======================================
  Files          97       97           
  Lines        3145     3145           
=======================================
  Hits         2851     2851           
  Misses        294      294           

Impacted Files	Coverage Δ
dart/lib/src/sentry_tracer.dart	92.10% <100.00%> (ø)

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