Support Custom Phone Authentication Providers

Currently, phone authentication supports providers like Twilio, MessageBird, Textlocal, and Vonage. While these are solid options, they can be limiting due to pricing, regional coverage, and country-specific support.

To improve flexibility, allow users to define custom phone authentication providers*. for example via Edge Functions or a similar mechanism. Users could implement standardized functions such as send_otp and verify_otp that behave like the built-in providers.

This approach would:
• Enable integration with any SMS provider that offers a REST API
• Reduce dependency on a fixed set of vendors
• Improve global coverage and cost optimization
• Give developers full control over their authentication pipeline