[FEATURE] LDAP connection with certificate/key?

[plaes]

Is your feature request related to a problem? Please describe.
Currently, only option to connect to LDAP seems to be by using password/username. It should be possible authenticate to external LDAP server by using client certificate (using certificate and key).

Describe the solution you'd like
Add possibility to use client certificate authentication.

Describe alternatives you've considered

I tried to connect to Google LDAP service using password/username, but apparently it is not working as expected and it's not really a recommended way either.

Additional context

Ldap library already should have support for this, by using DialURL method:

cert, err := tls.LoadX509KeyPair(certFile, keyFile)
if err != nil {
	return nil, err
}

certs := []tls.Certificate{cert}
tlsConfig := &tls.Config{
	ServerName:   serverName,
	Certificates: certs,
}

ldap.DialURL(url, ldap.DialWithTLSConfig(tlsConfig));

[plaes]

I have a WIP branch with certificate loading and connection initialization basically implemented.

But, I'm not sure whether ldap.Bind("","") in LDAP library actually supports the mTLS usecase, as it fails with:

LDAP Result Code 206 \"Empty password not allowed by the client\": ldap: empty password not allowed by the client

Apparently I need to use ExternalBind(). And it works!

[plaes]

Initial implementation is now ready for review at #509

[steveiliop56]

Implemented in #509. Thank you @plaes