feat: add support for custom provider configuration in the user config

[kshern]

What

• Add support for loading and merging custom provider configurations from the config 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

• Lint passes for changed files
• Tests pass for all files
• Documentation/comments updated as needed

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[kshern]

I have read the CLA Document and I hereby sign the CLA

[tibo-openai]

@kshern Could you take a look at the CI failures and then I will have a proper read through the implementation before commenting further. Thank you!

[bolinfest]

We are planning to have a unified config, so could you update this PR so that the changes apply to StoredConfig instead?

[tibo-openai]

@kshern Will merge once it's a single config and CI issue have been fixed.

[kshern]

We are planning to have a unified config, so could you update this PR so that the changes apply to StoredConfig instead?

Thanks for your patience! I've now completed the refactor to integrate the custom provider logic into StoredConfig as discussed.

@kshern Will merge once it's a single config and CI issue have been fixed.

and CI issue have been fixed

The PR should now be ready for another look. Please let me know if there's anything else needed. Thanks again for your guidance!

[tibo-openai]

Fixed merge conflicts and adjusted code structure and comments a bit. This is now going in, thank you for making your first contribution to the project, keep them coming!