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/models.rs

@@ -94,3 +94,14 @@ pub struct Genre {
     id: i32,
     name: String,
 }
+
+#[derive(Debug, Georm, PartialEq, Eq, Default)]
+#[georm(table = "UserRoles")]
+pub struct UserRole {
+    #[georm(id)]
+    pub user_id: i32,
+    #[georm(id)]
+    pub role_id: i32,
+    #[georm(defaultable)]
+    pub assigned_at: chrono::DateTime<chrono::Utc>,
+}