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/src/model_provider_info.rs

@@ -29,7 +29,7 @@ pub enum WireApi {
 }
 
 /// Serializable representation of a provider definition.
-#[derive(Debug, Clone, Deserialize, Serialize)]
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq)]
 pub struct ModelProviderInfo {
     /// Friendly display name.
     pub name: String,