enh: refactor setExtra and removeExtra to FFI/JNI

[buenaflor]

📜 Description

Part of moving away from method channels

💡 Motivation and Context

Closes #3204

💚 How did you test it?

Manual, integration tests

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[github-actions]

🚨 Detected changes in high risk code 🚨

High-risk code has higher potential to break the SDK and may be hard to test. To prevent severe bugs, apply the rollout process for releasing such changes and be extra careful when changing and reviewing these files:

• packages/flutter/android/src/main/kotlin/io/sentry/flutter/SentryFlutterPlugin.kt

[github-actions]

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

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

### Features

- refactor `setExtra` and `removeExtra` to FFI/JNI ([#3314](https://github.com/getsentry/sentry-dart/pull/3314))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description or adding a skip-changelog label.

Generated by 🚫 dangerJS against 19bd02d

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (enh/ffi-jni-tags-set@8dcc1d0). Learn more about missing BASE report.

Additional details and impacted files

@@                   Coverage Diff                   @@
##             enh/ffi-jni-tags-set    #3314   +/-   ##
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  Coverage                        ?   89.57%           
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  Files                           ?       99           
  Lines                           ?     3406           
  Branches                        ?        0           
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  Hits                            ?     3051           
  Misses                          ?      355           
  Partials                        ?        0           

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