This diff adds the prototype websteps implementation that used
to live at https://github.com/ooni/probe-cli/pull/506.
The code is reasonably good already and it's pointing to a roaming
test helper that I've properly configured.
You can run websteps with:
```
./miniooni -n websteps
```
This will go over the test list for your country.
At this stage the mechanics of the experiment is set, but we
still need to have a conversation on the following topics:
1. whether we're okay with reusing the data format used by other
OONI experiments, or we would like to use a more compact data
format (which may either be a more compact JSON or we can choose
to always submit compressed measurements for websteps);
2. the extent to which we would like to keep the measurement as
a collection of "the experiment saw this" and "the test helper
saw that" and let the pipeline choose an overall score: this is
clearly an option, but there is also the opposite option to
build a summary of the measurement on the probe.
Compared to the previous prototype of websteps, the main
architectural change we have here is that we are following
the point of view of the probe and the test helper is
much more dumb. Basically, the probe will choose which
redirection to follow and ask the test helper every time
it discovers a new URL to measure it w/o redirections.
Reference issue: https://github.com/ooni/probe/issues/1733
ba9151d4fa