Shared artifacts should be configurable to optionally fail the build or not

The artifact configuration for jobs should have an additional configuration option similar to the share/unshare link that defines whether the artifact will be "required" or "optional". A required artifact is expected to be created by the job, and if not created it is considered that the entire job has failed.
An optional artifact is a by-product the job will collect at the end of its run, but it may sometimes not be created and in such cases it is not considered as a failure.

This proposal is to mitigate the current behavior where artifacts not created or copied from remote agent to build server has no effect on the build status unless if it is a subscribed artifact.