Here we're refactoring stunreachability to not provide internally a
default input and to take in input an URL rather than a string.
The related ooni/spec change is https://github.com/ooni/spec/pull/227.
This diff has been extracted from https://github.com/ooni/probe-cli/pull/539.
Because the original diff was large, I'm splitting it in a set of
more easily manageable diffs.
The reference issue is https://github.com/ooni/probe/issues/1814, which
is complex enough to require us to proceed incrementally.
This diff WILL need to be backported to release/3.11.
cba72d1ca3