Accept depth as option for Recursive membership validator strategy constructor

This makes the `GitHub::Ldap::MembershipValidators::Recursive#perform` method signature identical to the other strategies, moving the options to the constructor to match the `GitHub::Ldap::MemberSearch` strategies too.

Technically this is a backwards incompatible API breakage but I don't think it's a significant enough change as to cause a major version bump since it's really fixing a bug with the API. Thoughts?

cc @jch 

I would document the `depth` attribute here.