Add Unbound as a provider

[vigneshsubbiah16]

Unbound is an AI Gateway that routes to several models across providers (OpenAI, Anthropic, Google, etc). It's already supported by other coding tools like Claude Code, Cursor, Roo Code, Cline, etc.

Adding Unbound would let users access multiple model providers through a single API key while keeping their existing gateway policies.

Implementation:

• Add Unbound to bundled providers with custom loader
• Fetch available models dynamically from /models endpoint
• Support UNBOUND_API_KEY env variable and config options

Docs: https://docs.getunbound.ai