Support beforeSendTransaction

[denrase]

📜 Description

Closes #1151

💚 How did you test it?

Added 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

@marandaneto There seem to be some oddities with serialization/deserialization with event properties when the instance is a transaction. Maybe it's just an issue in my test setup.

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- Support beforeSendTransaction ([#1238](https://github.com/getsentry/sentry-dart/pull/1238))

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Generated by 🚫 dangerJS against cb25b6a

[codecov]

Codecov Report

Base: 90.11% // Head: 90.13% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (b76b18e) compared to base (3e17cfa).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           v7.0.0    #1238      +/-   ##
==========================================
+ Coverage   90.11%   90.13%   +0.01%     
==========================================
  Files         164      164              
  Lines        5121     5130       +9     
==========================================
+ Hits         4615     4624       +9     
  Misses        506      506              

Impacted Files	Coverage Δ
dart/lib/src/sentry_options.dart	82.92% <ø> (ø)
dart/lib/src/sentry_client.dart	96.83% <100.00%> (+0.19%)	⬆️

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[marandaneto]

@denrase follow up about modules and fingerprint, since we are on the v7 branch, we can move them from the base class to the event class specifically, what do you think? since this is about the beforeSend, we can do it in a follow-up PR.

[denrase]

@marandaneto Yeah lets do this separate, if theres even something to do. Those are part of the event base class already, you can just never use them in the transaction subclass, but i guess they are not needed there.