richer input: riseupvpn as a use case

[bassosimone]

This issue is related to ooni/probe-cli#1125 and explores how it would look like to redesign the RiseupVPN experiment using the richer input (and, specifically, the DSL introduced in ooni/2023-05-richer-input#7. RiseupVPN is an interesting use case because this experiment is developed in collaboration with Riseup developers such as @cyBerta. We will consider this issue done when we'll have sketched out how RiseupVPN could look like when using richer input and we have gathered feedback on the strawman design. (This work goes hand in hand with the review of ooni/probe-cli#1125 and informs some of the cherry-picking choices we may apply in this respect.)