feat: introduce mcp-server crate (#792)

This introduces the `mcp-server` crate, which contains a barebones MCP
server that provides an `echo` tool that echoes the user's request back
to them.

To test it out, I launched
[modelcontextprotocol/inspector](https://github.com/modelcontextprotocol/inspector)
like so:

```
mcp-server$ npx @modelcontextprotocol/inspector cargo run --
```

and opened up `http://127.0.0.1:6274` in my browser:


![image](https://github.com/user-attachments/assets/83fc55d4-25c2-4497-80cd-e9702283ff93)

I also had to make a small fix to `mcp-types`, adding
`#[serde(untagged)]` to a number of `enum`s.

codex-rs/Cargo.toml

@@ -7,6 +7,7 @@ members = [
     "core",
     "exec",
     "execpolicy",
+    "mcp-server",
     "mcp-types",
     "tui",
 ]