feat: implement multi-database support and routing

[Einswilli]

This PR introduces comprehensive multi-database support to the Ryx ORM, allowing users to route queries and transactions across multiple database pools.

Key Changes

• Core Infrastructure: Replaced the global connection pool with a PoolRegistry to manage multiple named database aliases.
• Flexible Routing: Implemented a multi-layered routing system with the following priority:
  1. Explicit routing via .using(alias) on QuerySets and Managers.
  2. Dynamic routing via a configurable BaseRouter.
  3. Model-level defaults via Model.Meta.database.
  4. Fallback to the 'default' database.
• Multi-DB Transactions:
  • Added alias support to Ryx.transaction().
  • Implemented smart nesting: creates a SAVEPOINT when nesting on the same database, and a new independent transaction when switching databases.
• Compatibility: Maintained backward compatibility by allowing ryx_core.setup() to accept both a single URL string and a dictionary of aliases.
• Documentation: Added a new "Multi-Database Support" guide to the advanced documentation section.

Verification

• Verified via integration tests covering explicit, meta, and dynamic routing.
• Validated transaction behavior across multiple in-memory SQLite pools.