add a way to download a single file inside an artifact via the API
### Summary
When using the [License Compliance](https://docs.gitlab.com/ee/user/compliance/license_compliance/) feature to generate a [license scanning report artifact](https://docs.gitlab.com/ee/ci/yaml/index.html#artifactsreportslicense_scanning) there appears to be no mechanism to obtain the report artifact via [Job Artifacts API](https://docs.gitlab.com/ee/api/job_artifacts.html). Attempts to get the [named report as a single file](https://docs.gitlab.com/ee/api/job_artifacts.html#download-a-single-artifact-file-by-job-id) will 404.
A workaround is to generate the artifact using the `paths:` keyword, e.g.
```
license_scanning:
artifacts:
paths:
- gl-license-scanning-report.json
```
Which requires additional modifications to the pipelines including the License Compliance template, as well as incurring double the storage for the artifact.
### Steps to reproduce
1) [Include the License Scanning Template](https://docs.gitlab.com/ee/user/compliance/license_compliance/#configuration)
2) Attempt to retrieve the report artifact: `/api/v4/projects/<project_id>/jobs/<job_id>/artifacts/gl-license-scanning-report.json`
The result is a `404`.
### What is the current *bug* behavior?
The artifact report is not available via the API.
### What is the expected *correct* behavior?
The artifact report should be available via the API.
### Output of checks
This bug happens on GitLab.com and Self-Managed installs both.
### Additional Discussion
I'm not certain whether this should be considered a bug or a feature. This was a problem in the UI (and internal API?) when first released - and was discussed and addressed as part of this issue: https://gitlab.com/gitlab-org/gitlab/-/issues/218582 - as well as the related issue about the storage of report artifacts: https://gitlab.com/gitlab-org/gitlab/-/issues/23847
However, in Issue #218582 - there was a [comment](https://gitlab.com/gitlab-org/gitlab/-/issues/218582#note_347360804) that the `404` was also happening when attempting to get the report artifact via the API. There's an explicit acknowledgement that the work to do the fix likely wouldn't address the API use case and another issue would be needed. As far as I can tell, we didn't create that issue.
From initial issue description:
currently in http://docs.gitlab.com/ce/api/ci/builds.html it's only possible to download the whole artifacts file, however when browsing the artifacts file you could actually download a single file, too. it would be great if this would also be exposed as an api so that you could download it with a token.
### Possible workaround
See [comment below](https://gitlab.com/gitlab-org/gitlab/-/issues/15569#note_696814768)
issue