feat(system api): list instances by domains (#6806)

Allow to list instances by their domains on the system API.

closes #6785

proto/zitadel/instance.proto

@@ -71,6 +71,7 @@ message Query {
         option (validate.required) = true;
 
         IdsQuery id_query = 1;
+        DomainsQuery domain_query = 2;
     }
 }
 
@@ -83,6 +84,17 @@ message IdsQuery {
     ];
 }
 
+message DomainsQuery {
+    repeated string domains = 1  [
+        (validate.rules).repeated = {max_items: 20, items: {string: {min_len: 1, max_len: 100}}},
+        (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
+            max_items: 20,
+            example: "[\"my-instace.zitadel.cloud\", \"auth.custom.com\"]";
+            description: "Return the instances that have the requested domains";
+        }
+    ];
+}
+
 enum FieldName {
     FIELD_NAME_UNSPECIFIED = 0;
     FIELD_NAME_ID = 1;