Refactor ExecServer filesystem split between local and remote (#15232)

For each feature we have:
1. Trait exposed on environment
2. **Local Implementation** of the trait
3. Remote implementation that uses the client to proxy via network
4. Handler implementation that handles PRC requests and calls into
**Local Implementation**

codex-rs/exec-server/src/server.rs

@@ -1,4 +1,4 @@
-mod filesystem;
+mod file_system_handler;
 mod handler;
 mod processor;
 mod registry;