Rework custom fields handling

This patch changes a bit the handling of custom fields. Since in
general we use custom fields to aggregate multiple entries in the JSON
object into a safer data-type, we should also have a way to declare
which extra entries this field covers (so that in the future we can
say what are all the JSON keys for an object).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>