Add support for deleting job artifacts

## Description of the problem, including code/CLI snippet
Add support for deleting a job's artifacts, via the `delete-artifacts` API call. 

## Expected Behavior
https://docs.gitlab.com/ee/api/jobs.html#delete-artifacts

## Actual Behavior
Not implemented

## Specifications
https://docs.gitlab.com/ee/api/jobs.html#delete-artifacts
