Generate JSON schema for app-server protocol (#5063)

Add annotations and an export script that let us generate app-server
protocol types as typescript and JSONSchema.

The script itself is a bit hacky because we need to manually label some
of the types. Unfortunately it seems that enum variants don't get good
names by default and end up with something like `EventMsg1`,
`EventMsg2`, etc. I'm not an expert in this by any means, but since this
is only run manually and we already need to enumerate the types required
to describe the protocol, it didn't seem that much worse. An ideal
solution here would be to have some kind of root that we could generate
schemas for in one go, but I'm not sure if that's compatible with how we
generate the protocol today.

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

@@ -1,5 +1,9 @@
+mod export;
 mod jsonrpc_lite;
 mod protocol;
 
+pub use export::generate_json;
+pub use export::generate_ts;
+pub use export::generate_types;
 pub use jsonrpc_lite::*;
 pub use protocol::*;