Implemented custom SNI server name

[fpagliughi]

This implements #1580, to provide an optional, separate SNI server name.

It adds a new const char* serverName parameter to the SSL options. If it is non-NULL, then it is used for SNI validation, otherwise the host name is used. Thus the new capability is opt-in, and defaults to the previous implementation if not provided, and therefore is completely backward compatible.

[fpagliughi]

It took two tries, but this appears to be working now and passing all the unit tests.

The simplest test for this is to use the IP address in the URL and then set the serverName in the SSL options to the host name, and verify enabled like:

URL -> mqtts://5.196.78.28:8886
serverName -> test.mosquitto.org
verify -> 1

A thing to note is that this only allows for providing a single server name, even when using High Availability with multiple URLs. If you omit the server name, each host name will be derived from the URL, just like before, but if you do add a server name, all addresses will verify to that name.