fix(pluginsocket.proto): type for set_upstream

Fix the return type for the `.Service.SetUpstream` external plugin PDK
method.

Fixes issue Kong/go-pdk#114.

Sister PR: Kong/go-pdk#191

changelog/unreleased/kong/pluginsocket-proto-wrong-type.yml

@@ -0,0 +1,3 @@
+message: |
+  Fix an issue where external plugins using the protobuf-based protocol would fail to call the `kong.Service.SetUpstream` method with an error `bad argument #2 to 'encode' (table expected, got boolean)`.
+type: bugfix

kong/include/kong/pluginsocket.proto

@@ -328,7 +328,7 @@ service Kong {
     rpc Router_GetRoute(google.protobuf.Empty) returns (Route);
     rpc Router_GetService(google.protobuf.Empty) returns (Service);
 
-    rpc Service_SetUpstream(String) returns (google.protobuf.Empty);
+    rpc Service_SetUpstream(String) returns (Bool);
     rpc Service_SetTarget(Target) returns (google.protobuf.Empty);
 
     rpc Service_Request_SetScheme(String) returns (google.protobuf.Empty);