Add support for entities with composite primary keys using multiple
#[georm(id)] fields. Automatically generates {EntityName}Id structs for
type-safe composite key handling.
Features:
- Multi-field primary key detection and ID struct generation
- Full CRUD operations (find, create, update, delete, create_or_update)
- Proper SQL generation with AND clauses for composite keys
- Updated documNtation in README and lib.rs
Note: Relationships not yet supported for composite key entities
