varlink: Add new varlink_collect method

varlink: Add new varlink_collect method

varlink_collect is meant to be used when the client is willing to wait for the reply from the varlink method, much like varlink_call.
However, unlike varlink_call, it allows the client to collect all "more" replies that may be sent by method before the "final" reply is enqueued.
It aggregates all of these replies into a json variant array that it returns to the client.