Allow validation constraints to add in message replacements

We need the possibility to specify custom message replacement values when defining the validation constraints. This is need to support constraint messages as field API uses in the form of "%name: Some constraint message.", but %name cannot be retrieved generically.

Once #1969728: Implement Field API "field types" as TypedData Plugins is in, this should be changed to a bug report as this patch implements a work-a-round which double-translates messages. So then, that's something that needs to be fixed.