Send artifact uploads and downloads through ci-gateway

Due to HAProxy regression, our 307 redirect logic is broken. We need to eliminate the redirect. There are 2 options:

1. Proxy artifact uploads through ci-gateway.
2. Allow runner to have a separate upload URL configured, so that instead of the redirect, we go to gitlab.com egress directly.

This issue focuses on option 1. as a short-term mitigation, to allow us to re-enable ci-gateway for the rest of the traffic (git clones).

We may opt to keep the proxying in place, as it also cuts down on egress + cloudflare traffic, and the risk of bypassing cloudflare in this specific set of allowed paths seems manageable.

This ticket was created from INC-4485 and was automatically exported by incident.io 🔥