This commit starts to have the Payload POJOs replace the fields in
individual CollectionAdminRequest classes.  Backup is pretty much
finished.  I started on REstore and ran into a hiccup with the
collection-creation params - I could handle these via map operations,
but that's not very clean or strongly typed.  So my next step needs to
be creating a class that CreatePayload and REstorePayload can share with
various collection-creation params.

This eliminates some duplication between the API-specific classes,
provides an extra prod to make sure devs keep V2 APIs up to date, and
gives us a path forward for v1-v2 randomization on POJO-based
v2 APIs.