feat: add UserInfo request to JSON-RPC server (#3428)

This adds a simple endpoint that provides the email address encoded in
`$CODEX_HOME/auth.json`.

As noted, for now, we do not hit the server to verify this is the user's
true email address.

codex-rs/mcp-server/Cargo.toml

@@ -40,6 +40,7 @@ uuid = { version = "1", features = ["serde", "v4"] }
 
 [dev-dependencies]
 assert_cmd = "2"
+base64 = "0.22"
 mcp_test_support = { path = "tests/common" }
 os_info = "3.12.0"
 pretty_assertions = "1.4.1"