[VOL-1503] Add a GetMembership API to the core

This API is used by the Affinity Router to query the Go Core
for its membership info, it any.

Change-Id: I46aac8579a0452b665175cbd379876a702cce102
diff --git a/protos/voltha.proto b/protos/voltha.proto
index ef6eedf..7048e62 100644
--- a/protos/voltha.proto
+++ b/protos/voltha.proto
@@ -159,6 +159,13 @@
         };
     }
 
+    // Get the membership group of a Voltha Core
+    rpc GetMembership(google.protobuf.Empty) returns(Membership) {
+        option (google.api.http) = {
+            get: "/api/v1/membership"
+        };
+    }
+
     // Set the membership group of a Voltha Core
     rpc UpdateMembership(Membership) returns(google.protobuf.Empty) {
         option (google.api.http) = {