Openvpn experiment spec

[ainghazal]

Checklist

• [x] I have read the contribution guidelines
• [x] reference issue for this pull request: https://github.com/ooni/probe/issues/2202
• [x] related ooni/probe-cli pull request: https://github.com/ooni/probe-cli/pull/845
• [x] If I changed a spec, I also bumped its version number and/or date

Location of the issue tracker: https://github.com/ooni/probe

Description

This is a first draft of the openvpn experiment spec, in its current iteration.

[ainghazal]

Pushed a new commit addressing comments. I still have to ask @bassosimone for possible replacement of the several measurement archivals in use (I might be using outdated operation wrappers).

[ainghazal]

bumped to version 0.0.16; added ability to fetch arbitrary URLs.

the problem of the http archival format has been solved by using urlgetter. a minor modification is needed in urlgetter for this to happen (exposing a Dialer in the config).

[ainghazal]

closing now that we merged #293