add bind_s method

[ellis2323]

it's like simple_bind_s with a third argument. The third argument is the method used. A simple behaviour like simple_bind_s will be perfect.

[timabbott]

@ellis2323 thanks for your interest! If you'd like to contribute such a feature, I think it should be pretty doable. Just be sure to add a test like https://github.com/zulip/fakeldap/commit/042c1103de1ba297058901ccf18781990bfaebd5.