feat: Add option to specify the assignment rules for scaling policy

[mszostok]

Description

1. Should we nest as in assignment_rules, or keep the “duplicate” style like in labels_expressions?

   assignment_rules {
   	rules {
   		namespace {
   			names = ["default", "kube-system"]
   		}
   	}
   	rules {
   		workloads {
   			labels_expressions {
   				key      = "foo"
   				operator = "NotIn"
   				values   = ["a", "b"]
   			}
   			labels_expressions {
   				key      = "test"
   				operator = "Exists"
   			}
   		}
   	}
   }

   IMO the grouping in assignment_rules is more readable, subjects in our module are done this way too.
2. Should we map operators to Kubernetes label selector terms or keep the current ones?
   • e.g. KUBERNETES_LABEL_SELECTOR_OP_IN -> In, KUBERNETES_LABEL_SELECTOR_OP_NOT_IN -> NotIn, KUBERNETES_LABEL_SELECTOR_OP_EXISTS -> Exists, KUBERNETES_LABEL_SELECTOR_OP_DOES_NOT_EXIST -> DoesNotExist
     I know that we wanted to have it as close as possible to the API syntax, but who uses API directly? Maybe we can make it more user-friendly for the end users who will use the Terraform provider?
     With our great grcp->rest translation, is hard to have such naming.