feat: add openvpn experiment spec (#293)

## Checklist

- [x] I have read the [contribution
guidelines](https://github.com/ooni/spec/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request:
ooni/probe#2688
- [x] related ooni/probe-cli pull request:
ooni/probe-cli#1585
- [x] If I changed a spec, I also bumped its version number and/or date



## Description

Add initial spec for the `openvpn` experiment.
This proposal supersedes a previous version
(#255).
ICMP Pings and URLGrabber over the tunnel are not included, this
experiment just does OpenVPN handshakes at the moment.

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Co-authored-by: Simone Basso <[email protected]>