Feature request: allow clearing attachments in the global scope

[jpnurmi]

TLDR: Proposal to add sentry_clear_attachments() to help managing attachments in sentry-unreal and sentry-godot.

Originally posted by @tustanivsky in getsentry/sentry-unreal#982 (comment)

Given the discrepancies between Native, Android, and Cocoa in terms of attachment API support it looks like we have the following options:

1. Add clearAttachments method to Native which is already available for Android/Cocoa and expose that for global scope instead of giving user the ability to remove individual attachments
2. Add removeAttachment to Android/Cocoa to enable removing individual attachments on mobile
3. Combine 1 and 2 to cover all possible use cases
4. Leave only addAttachment for global scope without the ability to remove it
5. Leave things as-is and document the current limitations