Add a way to proxy upstream requests instead of external links

[benoitf]

For now, if you configure an upstream, it aggregates all results, including "local links" and "upstream links".

But let say you deploy a private instance of OpenVSX and only granting this instance to reach openvsx.org

If you've clients in your local area using the local instance of OpenVSX, openvsx.org results will be aggregated but it's possible that the clients may not have access/network access to reach openvsx.org directly.

So in that case it might be useful to have the local openvsx instance proxyfying all 'upstream requests'

Of course, it could be a flag to enable/disable and a local cache might help as well.

Original discussion happened there: https://github.com/eclipse/openvsx/pull/451#issuecomment-1128677134