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

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.

Start by tracing how the Thing Description represents affordances and how thing_slot-related affordances are handled. Define how an instance-dependent TD would carry metadata such as action button text and delayed property readback, then confirm that dynamic UI consumers can use it consistently. Done means the required affordance metadata and instance dependency are specified and supported.