initial mcp add interface (#3543)

Adds `codex mcp add`, `codex mcp list`, `codex mcp remove`. Currently writes to global config.

codex-rs/Cargo.lock

@@ -625,6 +625,7 @@ name = "codex-cli"
 version = "0.0.0"
 dependencies = [
  "anyhow",
+ "assert_cmd",
  "clap",
  "clap_complete",
  "codex-arg0",
@@ -637,7 +638,10 @@ dependencies = [
  "codex-protocol",
  "codex-protocol-ts",
  "codex-tui",
+ "predicates",
+ "pretty_assertions",
  "serde_json",
+ "tempfile",
  "tokio",
  "tracing",
  "tracing-subscriber",