Allow setting client certificate to `null`

[BalassaMarton]

Is your feature request related to a problem? Please describe.

When using Kerberos authentication and LDAPS protocol on Windows, with some (not all) LDAP server products, Bind fails with message:

Server Down. Result: 81. Method: ldap_connect

After some experimentation with Microsoft's DirectoryServices package, I've figured out that in this case, the client certificate should be set to null, which is currently not possible because of this line:
https://github.com/flamencist/ldap4net/blob/master/LdapForNet/Native/LdapNativeWindows.cs#L44
That should set certificateHandle to IntPtr.Zero and return false when the certificate is null.

Describe the solution you'd like

SetClientCertificate should accept null and handle this case properly in the native callback.

Describe alternatives you've considered

Workaround is to call SetOption , and pass a native callback that returns false:

connection.SetOption(
    Native.LdapOption.LDAP_OPT_CLIENT_CERTIFICATE,
    Marshal.GetFunctionPointerForDelegate<QUERYCLIENTCERT>(
        (IntPtr connection, IntPtr trustedCAs, ref IntPtr certificateHandle) =>
        {
            certificateHandle = IntPtr.Zero;
            return false;
        }));

Note: you have to copy the definition of QUERYCLIENTCERT because it's internal:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
internal delegate bool QUERYCLIENTCERT(IntPtr connection, IntPtr trustedCAs, ref IntPtr certificateHandle);

Additional context
This bug/limitation might be the root cause of #106