chore: replace machine filter with placement constraint in service spec

pkg/api/placement.go

@@ -0,0 +1,8 @@
+package api
+
+// Placement defines the placement constraints for service containers.
+type Placement struct {
+	// Machines is a list of machine names or IDs where service containers are allowed to be deployed.
+	// If empty, containers can be deployed to any available machine in the cluster.
+	Machines []string `json:",omitempty"`
+}