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.

️ This PR targets the branch of this PR

Closes https://github.com/sourcegraph/sourcegraph/issues/39616 and https://github.com/sourcegraph/sourcegraph/issues/39619

Test plan

New test is added.