Support providing a (set of?) URL(s) from a job to render as an external link in the job view
### Problem to solve
I have CI jobs which report to external resources (dashboards mainly). I'd like to provide a link from the job output or as an additional field to `artifacts` to read labeled URL(s) from a file to render on the sidebar of the job view page.
### Intended users
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
* [Devon (DevOps Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#devon-devops-engineer)
### Further details
External code coverage renderers/services (e.g., codecov.io), external dashboards collating results from external testing resources (e.g., CDash).
### Proposal
Something like
```yaml
jobname:
artifacts:
urls: path/to/url/file.txt
```
The file format is not of much consequence to me. Something simple like the `metrics.txt` is fine:
```
https://example.com/some-page Text describing the link
https://example.com/some-other-page Other text
```
### Permissions and Security
No different than to run a job. Links should be rendered with `nofollow` and any other "don't follow by default" attributes to prevent crawlers from inferring endorsement or the like.
### Documentation
Add documentation for the new mechanism to the CI YAML reference documentation.
### Testing
Probably not that much different than other artifact testing.
### What does success look like, and how can we measure that?
URLs for resources associated with jobs can be rendered from GitLab.
### What is the type of buyer?
CE. Doesn't seem like anything that would be enterprise-y to me. Not useful to our deployment without it being in CE (EE pricing can't work for us since we have open registration).
### Links / references
issue