enh: refactor setExtra and removeExtra to use FFI/JNI

[buenaflor]

📜 Description

Part of moving away from method channel

💡 Motivation and Context

Partly completes #3204

💚 How did you test it?

Manual, integration test

📝 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 use FFI/JNI ([#3303](https://github.com/getsentry/sentry-dart/pull/3303))

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 3cce35b

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.51%. Comparing base (6a99959) to head (3cce35b).

Additional details and impacted files

@@                     Coverage Diff                      @@
##           enh/ffi-jni-set-contexts    #3303      +/-   ##
============================================================
+ Coverage                     87.52%   89.51%   +1.98%     
============================================================
  Files                           195       98      -97     
  Lines                          6687     3376    -3311     
============================================================
- Hits                           5853     3022    -2831     
+ Misses                          834      354     -480     

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