yams #180461 maintain "RichString" type within the final schema [open]

As of today, RichString is a reader-only object that vinishes into a pair of Strings (data and meta-data).

We want to be able to tell from, e.g., cubicweb, that an attribute is of such a type without guessing.

prioritynormal
typebug
done in<not specified>
load1.000
load left1.000
closed by<not specified>