Configurable (always/never) renovate/artifacts status check

Describe the proposed change(s).

Allow users to opt into a setting which would result in the renovate/artifacts status check being always set, or never set.

We'd need a new option like artifactsCheckWhen which defaults to "failed" (today's behavior) and with a new option "always" which sets it for every Renovate commit, even if no artifacts are even included in the update, and another one "never", which skips it altogether.

Describe why we need/want these change(s).

This enables users to set this status check as a required check for all Renovate branches, or for those who wish to skip it.