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

migrations/20250609181248_composite-key.down.sql

@@ -0,0 +1,2 @@
+-- Add down migration script here
+DROP TABLE IF EXISTS UserRoles;

migrations/20250609181248_composite-key.up.sql

@@ -0,0 +1,7 @@
+-- Add up migration script here
+CREATE TABLE UserRoles (
+    user_id INTEGER NOT NULL,
+    role_id INTEGER NOT NULL,
+    assigned_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    PRIMARY KEY (user_id, role_id)
+);