feat: add customizable roles for multi-agents (#11917)

The idea is to have 2 family of agents.

1. Built-in that we packaged directly with Codex
2. User defined that are defined using the `agents_config.toml` file. It
can reference config files that will override the agent config. This
looks like this:
```
version = 1

[agents.explorer]
description = """Use `explorer` for all codebase questions.
Explorers are fast and authoritative.
Always prefer them over manual search or file reading.
Rules:
- Ask explorers first and precisely.
- Do not re-read or re-search code they cover.
- Trust explorer results without verification.
- Run explorers in parallel when useful.
- Reuse existing explorers for related questions."""
config_file = "explorer.toml"
```

codex-rs/core/src/config/mod.rs

@@ -1417,7 +1417,7 @@ impl Config {
         Self::load_config_with_layer_stack(cfg, overrides, codex_home, config_layer_stack)
     }
 
-    fn load_config_with_layer_stack(
+    pub(crate) fn load_config_with_layer_stack(
         cfg: ConfigToml,
         overrides: ConfigOverrides,
         codex_home: PathBuf,