feat: implement preliminary composite primary key support

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

tests/fixtures/composite_key.sql

@@ -0,0 +1,6 @@
+INSERT INTO UserRoles (user_id, role_id, assigned_at)
+VALUES
+    (1, 1, '2024-01-01 10:00:00+00:00'),
+    (1, 2, '2024-01-02 11:00:00+00:00'),
+    (2, 1, '2024-01-03 12:00:00+00:00'),
+    (3, 3, '2024-01-04 13:00:00+00:00');