feat: add codex completion to generate shell completions (#1491)

Once this lands, we can update our brew formula to use
`generate_completions_from_executable()` like so:


905238ff7f/Formula/h/hgrep.rb (L21-L25)

codex-rs/cli/Cargo.toml

@@ -17,6 +17,7 @@ workspace = true
 [dependencies]
 anyhow = "1"
 clap = { version = "4", features = ["derive"] }
+clap_complete = "4"
 codex-core = { path = "../core" }
 codex-common = { path = "../common", features = ["cli"] }
 codex-exec = { path = "../exec" }