Retrieve uploaded files using API
### Problem to solve
It's possible to upload a file using the api (`POST /api/v4/:project_id/uploads`) but there is no way to retrieve the uploaded files (it used to be possible using the Private-Token auth and the web route `/:namespace_id/:project_id/uploads/:secret/:file_name`)
Edit by @Andysoiron: It seems to be still possible to retrieve the file using `/:namespace_id/:project_id/uploads/:secret/:file_name` https://gitlab.com/gitlab-org/gitlab/issues/25838#note_272514463.
### Target audience
API users that want to retrieve uploaded files.
### Proposal
Add support to the REST API for retrieving a list of project uploads, downloading a specific uploaded file, and deleting an individual file.
#### Example Use Case
Add a new field to the response of `POST /projects/:id/uploads` to return the full path to the file including the project path.
Example:
```
{
"alt": "dk",
"url": "/uploads/66dbcd21ec5d24ed6ea225176098d52b/dk.png",
"full_url": "/namespace123/project321/66dbcd21ec5d24ed6ea225176098d52b/dk.png"
"markdown": ""
}
```
#### Acceptance
I can call the REST API to:
- [ ] Retrieve a paginated list of all uploads to a specific namespace.
- [ ] Download a specific uploaded file within a namespace.
- [ ] Delete an individual uploaded file.
### Issue Resolved details
API links for finding and managing uploads are here ([thanks Heinrich](https://gitlab.com/gitlab-org/gitlab/-/issues/25838#note_2013294498))!:
* https://docs.gitlab.com/ee/api/projects.html#markdown-uploads
* https://docs.gitlab.com/ee/api/groups.html#markdown-uploads
issue