[ONOS-7831] Implement GnmiHandshaker

Change-Id: I2232a724a86955483321f9fda571907aa2cb615a
(cherry picked from commit a884274c5796e6d8e6a58743d2d7f5ef47124b78)
diff --git a/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiClient.java b/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiClient.java
index 7f43d1f..242bc94 100644
--- a/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiClient.java
+++ b/protocols/gnmi/api/src/main/java/org/onosproject/gnmi/api/GnmiClient.java
@@ -57,5 +57,13 @@
      */
     CompletableFuture<SetResponse> set(SetRequest request);
 
+    /**
+     * Check weather the gNMI service is available or not by sending a
+     * dummy get request message.
+     *
+     * @return true if gNMI service available; false otherwise
+     */
+    CompletableFuture<Boolean> isServiceAvailable();
+
     // TODO: Support gNMI subscription
 }