feat: introduce --profile for Rust CLI (#921)

This introduces a much-needed "profile" concept where users can specify
a collection of options under one name and then pass that via
`--profile` to the CLI.

This PR introduces the `ConfigProfile` struct and makes it a field of
`CargoToml`. It further updates
`Config::load_from_base_config_with_overrides()` to respect
`ConfigProfile`, overriding default values where appropriate. A detailed
unit test is added at the end of `config.rs` to verify this behavior.

Details on how to use this feature have also been added to
`codex-rs/README.md`.

codex-rs/core/Cargo.toml

@@ -58,5 +58,6 @@ openssl-sys = { version = "*", features = ["vendored"] }
 [dev-dependencies]
 assert_cmd = "2"
 predicates = "3"
+pretty_assertions = "1.4.1"
 tempfile = "3"
 wiremock = "0.6"