Support reloptions of enum type

Support reloptions of enum type

All our current in core relation options of type string (not many,
admittedly) behave in reality like enums.  But after seeing an
implementation for enum reloptions, it's clear that strings are messier,
so introduce the new reloption type.  Switch all string options to be
enums instead.

Fortunately we have a recently introduced test module for reloptions, so
we don't lose coverage of string reloptions, which may still be used by
third-party modules.

Authors: Nikolay Shaplov, Álvaro Herrera
Reviewed-by: Nikita Glukhov, Aleksandr Parfenov
Discussion: https://postgr.es/m/43332102.S2V5pIjXRx@x200m