fix: Multiple attachments support for feedback

[tustanivsky]

📜 Description

Updated SentryFeedback to accept [Attachment] objects instead of raw [Data] arrays, enabling proper support for
multiple attachments with full metadata (filename, content type, etc.).

This also introduces some breaking changes to public API:

• init method signature
• dataDictionary() method now returns attachments as [[String: Any]] with metadata instead of [Data] affecting consumers of the onSubmitSuccess callback

This issue originally came up in Unreal SDK while adding attachment support for user feedback:

• Add attachments support for user feedback sentry-unreal#1121

💡 Motivation and Context

The previous implementation had several limitations:

1. Hardcoded metadata: attachmentsForEnvelope() assumed all attachments were screenshots and hardcoded filename: "screenshot.png" and contentType: "application/png"
2. Single attachment limitation: Only the first attachment was processed despite accepting an array
3. Lost metadata: Converting Attachment → Data → Attachment discarded important information like custom filenames and content types
4. Backend supports multiple: The Sentry backend can successfully process and display multiple feedback attachments, but the SDK wasn't taking advantage of this

💚 How did you test it?

• Added/updated tests
• TODO: Manually using Unreal SDK sample project.

📝 Checklist

You have to check all boxes before merging:

• 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.
• I updated the wizard 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.