feat: Add support for Firebase Auth as alternative to Google OAuth

[Om7035]

📋 Issue Description

Problem Statement

Currently, AuthKit only supports Google OAuth for social authentication. While Google OAuth is excellent for simple authentication, many developers want the flexibility to choose between:

1. Google OAuth - Lightweight, focused authentication
2. Firebase Auth - Full-featured auth with additional Firebase ecosystem benefits

This limitation forces developers to choose between AuthKit's simplicity or Firebase's comprehensive auth features.

Proposed Solution

Implement a provider selection system that allows users to choose between Google OAuth and Firebase Auth during setup, with:

• Configuration-time provider selection
• Seamless switching between providers
• Unified API regardless of backend provider
• Migration tools for existing installations

🎯 Use Cases & Benefits

When to Choose Google OAuth

• ✅ Simple auth needs - Just need user login/signup
• ✅ Minimal dependencies - No Firebase SDK overhead
• ✅ Fast setup - Quick configuration
• ✅ Cost-effective - No Firebase billing complexity
• ✅ Lightweight - Smaller bundle size

When to Choose Firebase Auth

• ✅ Firebase ecosystem - Already using Firestore, Storage, etc.
• ✅ Advanced features - Phone auth, anonymous auth, custom claims
• ✅ Real-time features - Firebase real-time database integration
• ✅ Analytics - Built-in user analytics and insights
• ✅ Scalability - Firebase's global infrastructure
• ✅ Enterprise features - SAML, multi-tenancy, custom domains

Business Value

• 📈 Broader adoption - Appeal to both simple and complex use cases
• 🎯 Market positioning - "One auth system, multiple backends"
• 🚀 Competitive advantage - More flexible than competitors
• 👥 User choice - Let developers pick what fits their stack