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/tui/src/cli.rs

@@ -17,6 +17,10 @@ pub struct Cli {
     #[arg(long, short = 'm')]
     pub model: Option<String>,
 
+    /// Configuration profile from config.toml to specify default options.
+    #[arg(long = "profile", short = 'p')]
+    pub config_profile: Option<String>,
+
     /// Configure when the model requires human approval before executing a command.
     #[arg(long = "ask-for-approval", short = 'a')]
     pub approval_policy: Option<ApprovalModeCliArg>,