This diff pins to ooni/probe-engine@3049779878
and starts using the recently introduced probe-engine APIs.
Namely, here, we use the InputLoader for loading URLs.
I've confirmed manually everything is still working as intended.
Part of https://github.com/ooni/probe/issues/1283.
(In particular, the InputLoader is the abstraction allowing us to load
input from several sources, including command line flags and external
files.)