Add Multi-Database Support

[ashbhati26]

Add Multi-Database Support

Problem Description

authpool is tightly coupled with MongoDB and Mongoose. Developers using other databases (e.g., PostgreSQL with Prisma, MySQL with Sequelize) cannot use this package without rewriting significant parts.

Proposed Solution

• Abstract database logic via an adapter pattern.
• Allow injecting a userAdapter in config:

startAuthServer({
  userAdapter: {
    findOrCreateUser: async (googleProfile) => {...},
    findUserById: async (id) => {...},
  }
});

Keep MongoDB as the default but fallback to adapter if provided.

Why this matter

Multi-database support makes authpool a framework-agnostic, future-proof solution. It can then serve any stack, from MERN to PostgreSQL-based backends.

Acceptance Criteria

• Supports a custom userAdapter object with required methods.
• MongoDB fallback works as default if adapter is not passed.
• Proper validation and error-handling if adapter is misconfigured.
• Examples included in README (e.g., Prisma + PostgreSQL usage).