feat: read model_provider and model_providers from config.toml (#853)

This is the first step in supporting other model providers in the Rust
CLI. Specifically, this PR adds support for the new entries in `Config`
and `ConfigOverrides` to specify a `ModelProviderInfo`, which is the
basic config needed for an LLM provider. This PR does not get us all the
way there yet because `client.rs` still categorically appends
`/responses` to the URL and expects the endpoint to support the OpenAI
Responses API. Will fix that next!

codex-rs/exec/src/lib.rs

@@ -66,6 +66,7 @@ pub async fn run_main(cli: Cli) -> anyhow::Result<()> {
             None
         },
         cwd: cwd.map(|p| p.canonicalize().unwrap_or(p)),
+        provider: None,
     };
     let config = Config::load_with_overrides(overrides)?;