oneclickexport: add REST API endpoint for archive download

Created by: sashaostrikov

This commit wires up one-click export ZIP archive creation code with REST API layer of frontend service. The endpoint should be accessed with POST HTTP method by site admins only.

Test plan: new unit test for REST API layer is added. Manually verified that the archive with required contents is downloaded on local sg instance.

Closes https://github.com/sourcegraph/sourcegraph/issues/39590