[app-server] add new account method API stubs (#5527)

These are the schema definitions for the new JSON-RPC APIs associated
with accounts. These are not wired up to business logic yet and will
currently throw an internal error indicating these are unimplemented.

codex-rs/protocol/src/lib.rs

@@ -1,3 +1,4 @@
+pub mod account;
 mod conversation_id;
 pub use conversation_id::ConversationId;
 pub mod config_types;