Replica.getRoleAsEnum exposes Protobuf generated classes in public API

The return type for Replica.getRoleAsEnum() in the Java client is org.apache.kudu.consensus.Metadata.RaftPeerPB.Role.

At a minimum we should mark this as InterfaceAudience.Private. If we do intend the method to be public API we should map it to a public enum owned by the client.