Adjust src/include/utils/jsonapi.h so it's not backend-only.

The major change here is that we no longer include jsonb.h into
jsonapi.h. The reason that was necessary is that jsonapi.h included
several prototypes functions in jsonfuncs.c that depend on the Jsonb
type. Move those prototypes to a new header, jsonfuncs.h, and include
it where needed.

The other change is that JsonEncodeDateTime is now declared in
json.h rather than jsonapi.h.

Taken together, these steps eliminate all dependencies of jsonapi.h
on backend-only data types and header files, so that it can
potentially be included in frontend code.