split extras/ping to its own package

[ainghazal]

extras/ping was forked after https://github.com/go-ping/ping, and modified to be able to send and receive IP packets over the "raw" vpn.Client (an openvpn connection).

We have decided to temporarily merge #35, that implements a different signature to use a Ping Connection that comes from wireguard code - discussion on a separated issue.

Independently of implementation and refactors, it is now clear that it would be useful to consider the Pinger as an independently useful component, that does not depend on vpn specific code. In that way we can maintain it on its own, and import it from any experiments that need to use it.