Support SASL security layer for client GSSAPI authentication

[vrlo]

The problem

To use gokrb5 with LDAP against Windows Active Directory, it
either requires channel binding with TLS, or SASL security layers for non-TLS
connections.

Aside from LDAP, other protocols like IMAP, SMTP, and Kafka support SASL/GSSAPI.

Enhancement proposal

This issue is for SASL security layers (integrity, confidentiality).

[Neustradamus]

@vrlo: Thanks for your ticket and PR:

• feat: Add SASL security layer support for GSSAPI #568