Add support of GSSAPI protocol connection

[PolliO]

Issue description

go-mysql-driver does not support connection by GSSAPI, which is used by Kerberos.

Example code

so, for now while I'm trying to connect to mysql via Kerberos user I failed with default case of auth() method in auth.go :

 default:
                errLog.Print("unknown auth plugin:", plugin)
                return nil, ErrUnknownPlugin
        }

for plugin - auth_gssapi_client

Configuration

Driver version (or git SHA):

Go version: run go version in your console : go1.15.8

Server version: E.g. MySQL 5.6, MariaDB 10.0.20

Server OS: E.g. Debian 8.1 (Jessie), Windows 10