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/mcp_server_config.rs

@@ -2,7 +2,7 @@ use std::collections::HashMap;
 
 use serde::Deserialize;
 
-#[derive(Deserialize, Debug, Clone)]
+#[derive(Deserialize, Debug, Clone, PartialEq)]
 pub struct McpServerConfig {
     pub command: String,