oneclickexport: add core export functionality
Created by: sashaostrikov
This is the first PR of 1-click export effort. This includes ExportRequest processing, site config fetching and redacting and creating a zip archive.
Closes https://github.com/sourcegraph/sourcegraph/issues/39616 and https://github.com/sourcegraph/sourcegraph/issues/39619
Test plan
New test is added.