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/proto/Cargo.toml

@@ -19,11 +19,8 @@ doctest = false
 [dependencies]
 anyhow.workspace = true
 collections.workspace = true
-futures.workspace = true
-parking_lot.workspace = true
 prost.workspace = true
 serde.workspace = true
-gpui.workspace = true
 
 [build-dependencies]
 prost-build.workspace = true