framework: Implement fetch-url-recursive-list and fetch-urlall-recursive-list

The recursive variants of fetch-url-list and fetch-urlall-list provide
a stable interface to a list of all the to-be-accessed URLs. That is
quite handy, e.g., when a list like that is necessary to implement
security policies.

Reviewed by: pizzamig
Approved by: portmgr (pizzamig)
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D40810