Add form field type to XEP-0157 form field
If form fields do no mention type, they are by default assumed
to be of type text-single. This causes problem in cases where
multiple form field values are included without mentioning type.
(https://xmpp.org/extensions/xep-0157.html#example-2).
However, field standardisation for the same xep-0157
(https://xmpp.org/extensions/xep-0157.html#registrar-formtype)
suggests that the form field type is list-multi. And so, it is
better if the server adds type to form fields as this saves the
client from first checking `FORM_TYPE` and determine form field
type as per standardisation .
Solves OF-2054
