webconnectivity 0.6: flag server side blocking

[bassosimone]

As part of the upcoming OONI Probe CLI v3.16.0 release, we're introducing incremental improvements to our web measurement methodology ("Web Connectivity"). We labelled these improvements "Web Connectivity v0.5" in #2237.

For v3.16.0, Web Connectivity v0.5 will only be available to miniooni users. We need additional research to confirm the improved experiment is working as intended. If everything is smooth, we expected v0.5 to be available to ooniprobe users as part of the v3.17.0 release cycle.

While v0.5 lands and stabilizes, it is worth starting to aim at subsequent changes. Drawing from our "websteps" research implementation (https://github.com/bassosimone/websteps-illustrated/), I would argue that one of the most important low hanging fruits to the Web Connectivity lineage is the possibility of flagging server side blocking. We should be able to flag possible server side blocking as an anomaly inside the probe and we should also support pipeline side flagging.