feat(dns): allow setting an externally managed cluster domain

pkg/api/dns.go

@@ -0,0 +1,8 @@
+package api
+
+// ClusterDomain is the domain name assigned to the cluster.
+type ClusterDomain struct {
+	Name string
+	// Reserved reports whether the domain is reserved in Uncloud DNS or set and managed externally.
+	Reserved bool
+}