This diff includes a first attempt at consolidating the patterns with
which we invoke OONI and third-party API calls.

I have refactored the code of httpx and httpapi into a new package
called httpclientx, added some tests, started converting some parts
of the tree, and explained myself in a design document.

Part of ooni/probe#2700

Spotted thanks to a very paranoid check inside ./internal/oonirun.

Was not a problem before for `httpx` because of its usage pattern and may
or may not be a problem for the `httpapi` package (did not check since
this work is focused on replacing both `httpx` and `httpapi`).

Previously, we were gracefully handling this case, but honestly it is
not the best approach to pretend there's an empty structure if the server
breaks the API and returns `"null"` rather than an object.

That said, it was still awesome to have this test in place because it
helped us to figure out this extra condition of which httpclientx should
be aware and that this problem needs to be dealt with systematically
inside the httpclientx package.

Conflicts:
	internal/enginelocate/cloudflare.go
	internal/enginelocate/ubuntu.go

Conflicts:
	internal/oonirun/v2_test.go

As before, here I am going to ensure there's redundancy.

Conflicts:
	internal/httpclientx/DESIGN.md
	internal/httpclientx/getraw.go
	internal/httpclientx/overlapped.go

Conflicts:
	internal/probeservices/login_test.go

Conflicts:
	internal/testingx/oonicollector.go

Conflicts:
	internal/probeservices/collector_test.go

Conflicts:
	internal/probeservices/bouncer.go
	internal/probeservices/collector.go
	internal/probeservices/login.go
	internal/probeservices/measurementmeta.go
	internal/probeservices/psiphon.go
	internal/probeservices/register.go
	internal/probeservices/tor.go

While there, add additional tests ensuring that we're actually
writing into the key-value store after check-in.

Also, ensure that failure to write into the key-value store
does not break check-in for good.

Conflicts:
	internal/legacy/mockable/mockable.go
	internal/mocks/session.go

Conflicts:
	internal/httpclientx/endpoint.go

Conflicts:
	internal/httpclientx/overlapped.go

Conflicts:
	internal/httpapi/sequence.go

Conflicts:
	internal/httpclientx/config_test.go
	internal/httpclientx/httpclientx_test.go