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/config_profile.rs

@@ -0,0 +1,15 @@
+use serde::Deserialize;
+
+use crate::protocol::AskForApproval;
+
+/// Collection of common configuration options that a user can define as a unit
+/// in `config.toml`.
+#[derive(Debug, Clone, Default, PartialEq, Deserialize)]
+pub struct ConfigProfile {
+    pub model: Option<String>,
+    /// The key in the `model_providers` map identifying the
+    /// [`ModelProviderInfo`] to use.
+    pub model_provider: Option<String>,
+    pub approval_policy: Option<AskForApproval>,
+    pub disable_response_storage: Option<bool>,
+}