Adding (partial) support for differential.querydiffs, and a timeout client option

[lopter]

Hello,

cc: @Pawka

Thank you for maintaining that library (and @jpoehls for the original implementation), I found it very useful while implementing a project at my company!

I had to make those two changes though:

1. Add a timeout option so that you can fail fast on the conduit calls;
2. Add request/response structs for differential.querydiffs, note that the structure is missing a couple fields that are very tricky to un-marshal, hope that's fine for now.

In the area of 1., another option I expect a need for is the ability to pass custom HTTP headers. I think it's reasonable at this time to add a new Header field (of type net.http.Header), but I'm wondering how well that approach of passing HTTP options via ClientOptions is going to scale, so I'm wondering if other people have an opinion here.

Let me know, if you have any question.

Best