chore: add a method to get a resource class by ID

Author: olevski

components/renku_data_services/crc/db.py

@@ -293,6 +293,13 @@ async def get_classes(
             orms = res.scalars().all()
             return [orm.dump() for orm in orms]
 
+    async def get_resource_class(self, api_user: base_models.APIUser, id: int) -> models.ResourceClass:
+        """Get a specific resource class by its ID."""
+        classes = await self.get_classes(api_user, id)
+        if len(classes) == 0:
+            raise errors.MissingResourceError(message=f"The resource class with ID {id} cannot be found", quiet=True)
+        return classes[0]
+
     @_only_admins
     async def insert_resource_class(
         self,