Add ability to set custom LDAP protocol

[stevebauman]

Closes #732

This PR adds the ability to set a set a custom protocol on LDAP connections in case developers need this control.

This can be set via the protocol configuration option:

use LdapRecord\Connection;

$connection = new Connection([
    'hosts' => ['192.168.1.1'],
    'protocol' => 'ldap://',
    'port' => 636,
]);