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/client/src/client.rs

@@ -22,7 +22,6 @@ use gpui::{actions, AppContext, AsyncAppContext, Global, Model, Task, WeakModel}
 use http_client::{AsyncBody, HttpClient, HttpClientWithUrl};
 use parking_lot::RwLock;
 use postage::watch;
-use proto::{AnyProtoClient, EntityMessageSubscriber, ProtoClient, ProtoMessageHandlerSet};
 use rand::prelude::*;
 use release_channel::{AppVersion, ReleaseChannel};
 use rpc::proto::{AnyTypedEnvelope, EnvelopedMessage, PeerId, RequestMessage};