TOP-1190: add enum value string hint

This adds an enum hint to the event handling of Tango on-change subscriptions. It uses metadata from the event notification to determine whether the return value is an Enum, and if it is, looks up the enum name (case-insensitively) in a list of known enums, which is currently a hand-picked selection from the SKA model, to try and get the string representation of the enum value. If this is found it is added to the debug message.