This PR starts to implement the refactoring described at https://github.com/ooni/probe/issues/1951. I originally wrote more patches than the ones in this PR, but overall they were not readable. Since I want to squash and merge, here's a reasonable subset of the original patches that will still be readable and understandable in the future.
Package github.com/ooni/probe-engine/probeservices
This package contains code to contact OONI probe services.
The probe services are HTTPS endpoints distributed across a bunch of data centres implementing a bunch of OONI APIs. When started, OONI will benchmark the available probe services and select the fastest one. Eventually all the possible OONI APIs will run as probe services.