Add project opt-in for sourcegraph integration

Discussing with @pslaughter about !16556 (merged), we end up questioning something. Should we add also an opt-in for Sourcegraph integration in projects?

I'm not completely aware of the workflow. I don't know if we tell Sourcegraph to index a project or if they already have the data.

I understand that public projects are available to anyone but it's not the same to tell explicitly to index a project.