feat: add support for custom provider configuration in the user config (#537)

### What

- Add support for loading and merging custom provider configurations
from a local `providers.json` file.
- Allow users to override or extend default providers with their own
settings.

### Why

This change enables users to flexibly customize and extend provider
endpoints and API keys without modifying the codebase, making the CLI
more adaptable for various LLM backends and enterprise use cases.

### How

- Introduced `loadProvidersFromFile` and `getMergedProviders` in config
logic.
- Added/updated related tests in [tests/config.test.tsx]


### Checklist

- [x] Lint passes for changed files
- [x] Tests pass for all files
- [x] Documentation/comments updated as needed

---------

Co-authored-by: Thibault Sottiaux <tibo@openai.com>

codex-cli/src/components/chat/terminal-chat.tsx

@@ -572,6 +572,7 @@ export default function TerminalChat({
         {overlayMode === "model" && (
           <ModelOverlay
             currentModel={model}
+            providers={config.providers}
             currentProvider={provider}
             hasLastResponse={Boolean(lastResponseId)}
             onSelect={(newModel) => {