Feature: new API for uploading directly from an URL

[alphakevin]

Sometimes user pastes HTML code with <img /> images from another site, and you want them static (the third party site may not served well in the future) and secure (some of the URLs are not HTTPS), so they should be uploaded directly from the original URLs and stored into NexusDocs

Request:

POST /api/namespaces/:namespace/upload-from-url

Body:

{
  urls: [
    '<url1>',
   // ...
  ],
}

Response:

{
  files: [
    {
       url: '<url1>',
       id: '<files_id1>',
    },
    // ...
  ],
}