Change Set state change function to filter instead of single namespace

This commit makes the setLastStateChangeToNow function take a list of
namespaces instead of a single namespace. If no namespaces is passed,
all namespaces will be updated. This means that the argument acts like a
filter.

namespaces.go

@@ -148,6 +148,21 @@ func (h *Headscale) ListNamespaces() ([]Namespace, error) {
 	return namespaces, nil
 }
 
+func (h *Headscale) ListNamespacesStr() ([]string, error) {
+	namespaces, err := h.ListNamespaces()
+	if err != nil {
+		return []string{}, err
+	}
+
+	namespaceStrs := make([]string, len(namespaces))
+
+	for index, namespace := range namespaces {
+		namespaceStrs[index] = namespace.Name
+	}
+
+	return namespaceStrs, nil
+}
+
 // ListMachinesInNamespace gets all the nodes in a given namespace.
 func (h *Headscale) ListMachinesInNamespace(name string) ([]Machine, error) {
 	err := CheckForFQDNRules(name)