feat(provider): add support for custom provider URLs

[tusharmath]

Description

This PR adds support for configuring custom provider URLs for OpenAI-compatible API providers including Open Router.

Changes

• Add 'open_ai_url' method to the Provider enum to allow setting custom URLs
• Refactor provider key functions to accept string slices for better memory efficiency
• Check for 'OPENAI_URL' environment variable to configure custom endpoints
• Update README with comprehensive documentation on provider configuration options
• Add examples of custom URLs configuration for different scenarios

Related Issues

• Addresses the need for connecting to self-hosted or enterprise OpenAI-compatible models
• Improves flexibility for users with custom API endpoints
• Supports proxy services and regional API deployments

Testing

• Tested with default Open Router endpoint
• Tested with custom URL configuration
• Verified backward compatibility

Breaking Changes

None. This is a backward-compatible enhancement that maintains all existing functionality.