add RtAudio backend

[BillyDM]

This adds an RtAudio backend and reworks the AudioBackend API to be compatible with it.

[CorvusPrudens]

Is it possible, given these changes, to get the set of all input and output devices for a given generic backend? It looks like a backend's enumerator doesn't need to implement any particular trait, so I think the answer is no.

This is a bit of a pain point for bevy_seedling, largely because it's nice DX for a user if changing the backend doesn't require propagating its generics throughout their code base. With the new setup, bevy_seedling itself would need to provide an abstraction (i.e. some trait) over backends and their device enumerators. The orphan rule means this abstraction couldn't be implemented by someone pulling in a foreign backend.

Maybe that's fine. I doubt anyone is currently bringing in a third-party backend that bevy_seedling wouldn't know about. But it does raise the question of where this abstraction should take place. At the very least, I'm curious what you think of this.