Add gssapi plugin auth

[PolliO]

Description

Added support for GSSAPI protocol, that now we can connect to mysql via Kerberos user from Linux server without password. Oppened issue - https://github.com/go-sql-driver/mysql/issues/1232

Checklist

• [ + ] Code compiles correctly
• [ - ] Created tests which fail without the change (if possible)
• [ + ] All tests passing
• [ - ] Extended the README / documentation, if necessary
• [ + ] Added myself / the copyright holder to the AUTHORS file

[dolmen]

@PolliO Please fix the tasks list in this PR description: use [ ] and [x].

[Joematpal]

What is that status of this? Is any help needed?