chore(corebluetooth): Use objc2 and its framework crates

[madsmtm]

objc2 is the successor to objc, which improves the way memory-management is done using the objc2::rc::Id type, which is effectively a typed StrongPtr. Additionally it provides the msg_send_id! macro, which ensures that the Objective-C memory management rules are followed correctly. Finally, it has encoding verification when debug assertions are enabled, which verify the calling convention at runtime, and can catch errors with using an incorrect method signature. The second commit fixes the errors that was found with this.

Compared to cocoa, objc2-foundation provides type-safe wrappers around each class, which means that you can't accidentally call e.g. allKeys on something that is not a dictionary, and it allows us to naturally use Options for nullable values. The third commit replaces the manual interface to Foundation types in src/corebluetooth/framework.rs with the types from objc2-foundation.

This PR is only the first part of the transition; in the future we will have objc2-core-bluetooth, see madsmtm/objc2#608, which should eliminate the need all of src/corebluetooth/framework.rs. But in the meantime, this still improves the safety significantly.

Feel free to ask if something is unclear!

[qdot]

Wow, thanks so much for this! The macOS portion of the codebase is probably the least understood part (I really just ripped out what Servo had and poked it until it worked), so I'm happy to see some maintenance happening to hopefully make it safer and more maintainable. I'll try to review this and get it in soon.