Vendor LiveKit protocol (#11672)

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

crates/live_kit_server/vendored/protocol/livekit_webhook.proto

@@ -0,0 +1,36 @@
+syntax = "proto3";
+
+package livekit;
+option go_package = "github.com/livekit/protocol/livekit";
+option csharp_namespace = "LiveKit.Proto";
+option ruby_package = "LiveKit::Proto";
+
+import "livekit_models.proto";
+import "livekit_egress.proto";
+import "livekit_ingress.proto";
+
+message WebhookEvent {
+  // one of room_started, room_finished, participant_joined, participant_left,
+  // track_published, track_unpublished, egress_started, egress_updated, egress_ended, ingress_started, ingress_ended
+  string event = 1;
+
+  Room room = 2;
+
+  // set when event is participant_* or track_*
+  ParticipantInfo participant = 3;
+
+  // set when event is egress_*
+  EgressInfo egress_info = 9;
+
+  // set when event is ingress_*
+  IngressInfo ingress_info = 10;
+
+  // set when event is track_*
+  TrackInfo track = 8;
+
+  // unique event uuid
+  string id = 6;
+
+  // timestamp in seconds
+  int64 created_at = 7;
+}