Add support for feedback envelope header item type

[denrase]

📜 Description

Adds the feedback envelope header item type

💡 Motivation and Context

Feedback envelope items are just regular envelope items, with additional context data and support for attachments.

Currently we are implementing this on flutter, and android is the only platform missing, as the type is lost from the envelope sent to the native sdk from the flutter side.

Relates to getsentry/sentry-dart#2230 (comment)

💚 How did you test it?

Added unit tests.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

[github-actions]

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

Generated by 🚫 dangerJS against 598b002

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	419.06 ms	454.98 ms	35.92 ms
Size	1.70 MiB	2.35 MiB	661.04 KiB

[stefanosiano]

nice!