This PR vendors the protobuf files from the LiveKit protocol so that we
don't need to have that entire LiveKit protocol repo as a submodule.

---

Eventually I would like to replace this with the
[`livekit-protocol`](https://crates.io/crates/livekit-protocol) crate,
but there is some churn that needs to happen for that.

The main problem is that we're currently on a different version of
`prost` used by `livekit-protocol`, and upgrading our version of `prost`
means that we now need to source `protoc` ourselves (since it is no
longer available to be compiled from source as part of `prost-build`).

Release Notes:

- N/A