Support richer input - explorer

[bassosimone]

This issue is about extending explorer 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 explorer changes. We can consider this issue "done" once the planned changes have been implemented, documented, merged to ooni/explorer and deployed the changes.