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/simple_struct.rs

@@ -143,12 +143,12 @@ async fn delete_by_id_should_delete_only_one_entry(pool: sqlx::PgPool) -> sqlx::
     let id = 2;
     let all_authors = Author::find_all(&pool).await?;
     assert_eq!(3, all_authors.len());
-    assert!(all_authors.iter().any(|author| author.get_id() == &id));
+    assert!(all_authors.iter().any(|author| author.get_id() == id));
     let result = Author::delete_by_id(&pool, &id).await?;
     assert_eq!(1, result);
     let all_authors = Author::find_all(&pool).await?;
     assert_eq!(2, all_authors.len());
-    assert!(all_authors.iter().all(|author| author.get_id() != &id));
+    assert!(all_authors.iter().all(|author| author.get_id() != id));
     Ok(())
 }