Make more methods of MailServer protected / public

[m-gallesio]

There is a dedicated way to make a SmtpClient use a custom certificate validation callback:
https://mimekit.net/docs/html/P_MailKit_MailService_ServerCertificateValidationCallback.htm

It is easy enough to implement a custom logic using the default as a model:

MailKit/MailKit/MailService.cs

Line 545 in 449d58a

protected static bool DefaultServerCertificateValidationCallback (object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)

However some components of the default function (IsUntrustedRoot and IsKnownMailServerCertificate) which would / could be repeated verbatim cannot be reused due to being private / internal.

Our current approach uses a custom subclass of SmtpClient and needs of the two the logic in IsUntrustedRoot, which is short enough to copy and adapt which however means replicating something that already exists in the library. We do not need IsKnownMailServerCertificate, and were us (or others) to need it it would be considerably more unwieldy to repeat.

I lack the experience to properly evaluate which of the many other components could be useful to subclasses, but I feel at least the two I noticed could be exposed.

[jstedfast]

To be honest, I've been planning to kill off IsKnownMailServerCertificate and if that goes away, there's no point in keeping IsUntrustedRoot.

Now that everyone is using dotnet (as opposed to mono) on Linux/Mac, it seems like these hacks aren't really needed anymore.