Allow Drupal admins to add individual Brandfolder files to Drupal's file collection

Problem/Motivation

Currently, images from a linked Brandfolder can be pulled into Drupal and associated with any Drupal entity via an image field. When that happens, a Drupal managed file will be created, and that file [entity] can be managed via Drupal's file admin tools. However, it is not possible to add Brandfolder-hosted files to Drupal on their own (i.e. not linked to any Drupal entity). This could be a useful feature for certain workflows.

Proposed resolution

Provide a Drupal-based mechanism for adding Brandfolder files to Drupal ad hoc. This would appear in the admin menu under "Content > File > Add" E.g. at the /file/add/brandfolder path.

Remaining tasks

Create a local menu task for /file/add/brandfolder, with a variant of the standard file upload form that uses a Brandfolder Browser form element to allow selection of a Brandfolder asset.
Ensure all necessary validation and processing occurs.

User interface changes

New option in admin menu: "Content > File > Add File > Brandfolder."
Simple form with Brandfolder Browser to allow selection of a Brandfolder asset.
Files added in this manner will behave the same way as files added via a Brandfolder Browser widget associated with an image field, etc.

API changes

None.

Data model changes

None.

Release notes snippet

Ability to add Brandfolder-hosted files to Drupal's managed file collection directly (not necessarily attached to an entity via an image or file field).