Add support for SQL Server

[rgl]

SQL Server does not uses a raw TLS connection, instead, it uses TLS over the TDS protocol, as such, we need to use the https://github.com/denisenkom/go-mssqldb library. I'm making a patch at denisenkom/go-mssqldb#592 that adds support for configuring the TLS client, when that is merged, it should enable us to use it in certigo.

Should I prepare a PR that adds support for SQL Server to certigo? What do you think?