oneclickexport: add external_service_repos table DB query processor

Created by: sashaostrikov

This commit also adds a function to list external service repos to the store.

Closes https://github.com/sourcegraph/sourcegraph/issues/39947 and https://github.com/sourcegraph/sourcegraph/issues/39960

Test plan

New tests added both for store and one-click export code.