[macOS, sandbox] Implement optional native file selection dialog support for sandboxed apps.

[bruvzg]

Alternative to #47056, implements godotengine/godot-proposals#1123 on macOS.

Adds ability to access arbitrary folders for macOS sandboxed apps (Entitlements → App Sandbox → Enabled is selected during export).

• Adds files.user-selected entitlement support to the export dialog.
• Adds OS.is_sandboxed() to detect running in the macOS sandbox.
• Adds DisplayServer.file_dialog_show() to display native file selection dialog. Which also saves security-scoped bookmarks for selected files/folders, to automatically gain access in the next session.
• Adds FileDialog.use_native_dialog property to seamlessly integrate native dialog to existing apps, disabled by default, used automatically when sandboxed app is accessing file system.
• Implements OS_OSX::get_granted_permissions() to get list of accessible folders/files.
• Adds OS.revoke_granted_permissions() to clear list of accessible folders/files.
• Can be used in non-sandboxed apps.

[akien-mga]

Seems good to me overall, but let's keep it for 4.1 since we're now at RC.

My main concern with this PR is not specific to the changes here, but more to my ever growing feeling that we have too many single-platform-specific APIs in OS/DisplayServer which make these abstract APIs harder to parse IMO.

I'd like a system where we can easily add platform-specific features to platform singletons (MacOS, Android, etc.) so we don't have to make all these behaviors virtual in the abstract class, to implement them only on one platform. But that's outside the scope of this PR completely, and so far it's almost mostly my personal pet peeve, not necessarily a widespread consensus that this is how we should do this.

[YuriSizov]

@bruvzg This needs a rebase and addressing Remi's documentation notes, please 🙏

[YuriSizov]

Thanks!