[app-server] refactor: split API types into v1 and v2 (#6005)

Makes it easier to figure out which types are defined in the old vs. new
API schema.

codex-rs/app-server-protocol/src/lib.rs

@@ -6,4 +6,6 @@ pub use export::generate_json;
 pub use export::generate_ts;
 pub use export::generate_types;
 pub use jsonrpc_lite::*;
-pub use protocol::*;
+pub use protocol::common::*;
+pub use protocol::v1::*;
+pub use protocol::v2::*;