Add ssh protocol probe (minimal, no command)

[hv15]

Hi there, I really like this project and have presented it to colleagues at work as a nice solution to trivially check on status of various machines. I've however found a limitation, that this PR tries to close. We have a lot of machines where the only network visible endpoint is the SSH port (running sshd); we also don't have ICMP activated, this is actively blocked by our firewall. Using the TCP probe is reasonable, but it causes a lot of noise in the sshd logs due to the connection not performing a suitable handshake with the daemon.

This PR provides an implementation to handle connecting to an SSH port, and resolve the handshake correctly, without needing to authenticate. This keeps the sshd logs clean, and means that the probe itself also checks that it's connecting to a working sshd. With TCP it's still possible to connect to an open port, and thus possible not report an issue, even if the listening process is in an unstable state

Note, this PR does not provide a completely new probe that supports SSH interactivity, e.g. running a command on a remote machine. Doing this is generally possible, but I think would need a more significant and careful refactoring of the probe module.

Some details:

• provide SSH probe, can be set with ssh://<address>:22 (similar to the TCP probe)
• built on the ssh2 crate

[valeriansaliou]

Thank you so much for the PR! I'll process it when I'm batching work on Vigil. LGTM to me checking the diff real quick.