search: add context to Go Search resolver

Created by: rvantonder

So I wrote that refactor in https://github.com/sourcegraph/sourcegraph/pull/12086 but it's mostly unneeded because it turns out I can just do this (discovered in a draft PR @stefanhengl put up and then I validated that ctx has the expected info). I really wish it was more obvious that the ctx is an optional thing based on the GO GQL project description but OK.

I will port the smaller useful bits in #12086 in a separate PR tomorrow.