[ONOS-7141][ONOS-7142] Add GnmiClient and GnmiController

Change-Id: I6347da378d9e4b59a38e2efacc27599b96dc1515
diff --git a/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java b/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java
new file mode 100644
index 0000000..f4964ed
--- /dev/null
+++ b/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiController.java
@@ -0,0 +1,28 @@
+/*
+ * Copyright 2018-present Open Networking Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.onosproject.gnmi.api;
+
+import com.google.common.annotations.Beta;
+import org.onosproject.grpc.api.GrpcClientController;
+
+/**
+ * Controller of gNMI devices.
+ */
+@Beta
+public interface GnmiController
+        extends GrpcClientController<GnmiClientKey, GnmiClient> {
+}