probeservices: cleanup and refactoring to prepare for richer input

[bassosimone]

To do ooni/ooni.org#1292, we first need to do a pass of cleanup and refactoring of the probeservices package, which is where we will need to implement the change. Here are some of the activities we should perform:

• probeservices: sync up data formats with ooni/api  #2372
• probeservices: make sure we can get a compressed check-in response #2379
• use internal/httpapi rather than internal/httpx (which allows for automatic fallback)
• engine: bump orchestra.json version number #2342
• implementing caching for results returned by the check-in API
• probeservices: drop deprecated APIs #2380
• check-in: return test helpers info and deprecate /api/v1/test-helpers #2392
• make sure we use the check-in API as the preferred geolocation mechanism
• probeservices: A/B testing using check-in API feature flags #2396 (needed by Create throttling measurement methodology ooni.org#1296)

Once these cleanups have been implemented, we will have simplified the implementation that needs to be modified to deal with richer input (hence reducing the impact of the change) thus creating the optimal conditions for introducing a richer input.

[bassosimone]

The scope of this issue is probably too broad. I think it is better to spec out a more narrow issue, such as #2597, which defines a very clear goal, and then eventually fork off child issues if it seems #2597 is too broad. Roughly speaking, these two issues call for the ~same changes, but I'd rather to use #2597 instead because it is dependent on a concrete improvement, while this one is too broad. (This issue was originally an Epic, which is why was too broad, but we're trying to work on smaller issues and less Epics now, which is why I'd like to replace this issue with a more narrow one and I do not feel super happy about rewriting the content of this issue entirely when a new issue would be more clear.)