feat: Add support for external SASL binds

[timoguin]

go-ldap currently only supports simple binds. Add support for external SASL binds to use Kerberos authentication.

Depends on exposing TLS config options via #5.

References:

• https://pkg.go.dev/github.com/go-ldap/ldap/v3#Conn.ExternalBind
• https://pkg.go.dev/github.com/go-ldap/ldap/v3#example-Conn.ExternalBind