Add a way to attach more metadata to affordances and/or things

[rwb27]

At the moment, the implementation of dynamic UI in OpenFlexure needs more metadata for each affordance than is currently included in the Thing Description. If we could specify things like button text (for an action) or whether we should read back the value with a delay after setting (for a property) that feels like the sort of thing that would sit naturally in the affordance's metadata in the TD.

I think this makes more sense if the Thing Description can depend on the Thing instance - but I also think that's a good idea anyway because of thing_slot related affordances.