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/assistant/Cargo.toml

@@ -65,6 +65,7 @@ proto.workspace = true
 regex.workspace = true
 release_channel.workspace = true
 rope.workspace = true
+rpc.workspace = true
 schemars.workspace = true
 search.workspace = true
 semantic_index.workspace = true

crates/assistant/src/context_store.rs

@@ -2,7 +2,6 @@ use crate::{
     prompts::PromptBuilder, Context, ContextEvent, ContextId, ContextOperation, ContextVersion,
     SavedContext, SavedContextMetadata,
 };
-use ::proto::AnyProtoClient;
 use anyhow::{anyhow, Context as _, Result};
 use client::{proto, telemetry::Telemetry, Client, TypedEnvelope};
 use clock::ReplicaId;
@@ -16,6 +15,7 @@ use language::LanguageRegistry;
 use paths::contexts_dir;
 use project::Project;
 use regex::Regex;
+use rpc::AnyProtoClient;
 use std::{
     cmp::Reverse,
     ffi::OsStr,