Add support for checking the fingerprint of self-signed SSL certs

[sinbad]

Discussed with @novabyte on Gitter, sounds like this could be useful.

I wanted to have some checking of SSL certs even though I was using a self-signed certificate, so I've added the ability to provide a list of cert fingerprints that are considered acceptable. Example usage:

        var client = new NClient.Builder(sb.ToString())
            .Host("your.host.com")
            .Port(443)
            .SSL(true)
            .SSLAcceptAllCertificates(false)
            .SSLValidKeyFingerprints(new string[] { "9E10ACEB4DD13F0FD3E187C3CBC270605EA65031" })
            .Build();

It defaults to the previous behaviour to accept all certificates so as not to break anything, although I really think the default should be the opposite at some point to encourage better security.

Hope it's useful!

[MarcusRiemer]

Is there any ETA for this? Or any reason why this shouldn't be merged?

[novabyte]

@MarcusRiemer Since the launch of the new client I've separated out this work into a separate commit which will be completed tomorrow. And incorporated as part of the release. 😄

[novabyte]

This was added in the 2.1.0 release.