Move ProtoClient to RPC crate, behind feature flag disabled in collab (#17908)

This fixes a bug where we accidentally added a `gpui` transitive
dependency in `collab`.

Release Notes:

- N/A

crates/channel/src/channel_buffer.rs

@@ -5,8 +5,8 @@ use collections::HashMap;
 use gpui::{AppContext, AsyncAppContext, Context, EventEmitter, Model, ModelContext, Task};
 use language::proto::serialize_version;
 use rpc::{
-    proto::{self, AnyProtoClient, PeerId},
-    TypedEnvelope,
+    proto::{self, PeerId},
+    AnyProtoClient, TypedEnvelope,
 };
 use std::{sync::Arc, time::Duration};
 use text::BufferId;

crates/channel/src/channel_chat.rs

@@ -11,7 +11,7 @@ use gpui::{
     AppContext, AsyncAppContext, Context, EventEmitter, Model, ModelContext, Task, WeakModel,
 };
 use rand::prelude::*;
-use rpc::proto::AnyProtoClient;
+use rpc::AnyProtoClient;
 use std::{
     ops::{ControlFlow, Range},
     sync::Arc,