Introduce new check method ("SSL")

[JFronny]

I use Upptime to check on a variety of services I run on my home server.
One of the services I use it for is a mail server.
Unfortunately, the simple "tcp-ping" check does not work in this use case since the port forwarding configured in our network causes the check to assume the port is accessible (and thus that the service is working) even if it is down.
Opening an SSL connection and checking whether the returned certificate is correct avoids this issue.
Additionally, this check is undoubtedly useful for any other services that support SSL but don't use HTTPS, allowing Upptime instances to verify a bit more of their configuration than "tcp-ping" would.

This code is largely based on the existing "tcp-ping"-check but altered to use a TLS socket.

[btlogy]

I would dare to give an opinion about the implementation (not my area of expertise), but it seems to be a nice feature @JFronny: 👍

Now LetsEncrypt is ending their expiration notification, I was looking if Upptime had some advanced TLS capabilities...

And by looking at you PR, I am under the impression that this check-tls method could also provide a better way to the check if the certificate is about to expire via getPeerCertificate(), instead of parsing curl verbose output in the check-http method).

Am I right to think this way?

Also, I suppose you should create an issue to document this check-tls use case and get this PR picked up for review? And I'll likely create one there for the certificate validity...