Support richer input - backend

[bassosimone]

This issue is about extending the backend to support richer input. The main epic issue for this work is ooni/ooni.org#1295. Now that we have experimented with richer input in the probe and produced a bunch of experiments and PoCs, we are going to discuss and produce a design document (ooni/ooni.org#1295). In turn, the discussion and the design document will inform the required backend changes to support the probe and explorer needs. We can consider this issue "done" once the planned changes have been implemented, documented, merged to ooni/backend and deployed in the testing and production environments.

[bassosimone]

In the design document, we decided to keep check-in v1. There are fine grained issues about extending check-in v1 and adding support for richer input, such as:

• richer input: support dnscheck #754
• richer input: support signal #756
• richer input: support stunreachability #755
• richer input: support torsf #758
• check-in v1: richer input for riseupvpn: discuss protoype #757
• check-in v1: richer input for urlgetter: discuss prototype #759

As such, we can probably close this issue or, alternatively, bless it as an Epic @jbonisteel

[bassosimone]

Compared to #713 (comment), there has been a change of plans, where now we're going to take advantage of the backend microservices architecture and we won't need to modify the check-in API, with all the concerns that changing a core API for Web Connectivity brings with it.

Updated plan:

• richer input: support dnscheck #754
• richer input: support signal #756
• richer input: support stunreachability #755
• richer input: support torsf #758

Cc: @jbonisteel