[core] Set proto User.externalId to explicit presence

Author: teo

common/protos/common.pb.go

@@ -33,7 +33,7 @@ option go_package = "github.com/AliceO2Group/Control/common/protos;pb";
 
 message User {
   // The unique CERN identifier of this user.
-  int32 externalId = 1;
+  optional int32 externalId = 1;
   // The unique identifier of this entity.
   optional int32 id = 2;
   // Name of the user.