vpn: ship MVP for the openvpn experiment

[ainghazal]

Definition of done

• A release of OONI ships an OpenVPN experiment as part of the experimental suite.
• As an user, just by running experimental tests, I'm able to run one or more OpenVPN handshake attempts against a set of predetermined endnpoints, and see some form of preliminary outcome after executing these tests.
• Data containing results and detailed information about the operations is submitted and processed by the OONI backend.

As a first iteration, we'll be shipping a predetermined set of endpoints to contact and probe RiseupVPN infrastructure.

Due to constrains derived from credentials distribution, we also will need to be able to distribute valid credentials to probes.

After some analysis, the current consensus is that OpenVPN will be an experiment of its own, and that will eventually replace the existing RiseupVPN experiment. The previous API requests are better exercised by dedicated target lists in webconnectivity, and the new experiment has only to deal with protocol analysis and should not fail by unrelated API blocks or intermittencies.

This issue supersedes #2026.

[ainghazal]

As a first iteration, we'll be shipping a predetermined set of endpoints to contact and probe RiseupVPN infrastructure.

as part of the discussion and implementation of the backend service, we somehow went past the first definition of MVP and extended it so that the VPN probe service is also in charge of endpoint discovery and credential distribution.