Media Library backed by a Netlify CMS collection

**Is your feature request related to a problem? Please describe.**
As a Netlify CMS user, I would like the ability to present a media library where users can select from a curated collection of images, videos or other media items (potentially) not contained within git.

**Describe the solution you'd like**
When the media library component is opened, instead of displaying a list of media files contained in a media_folder, a list of items specified in a standard Netlify CMS collection would be displayed instead.

Collections of this type would need to adhere to a minimal base schema (id, url, but also potentially thumbnail, file size, etc) but could also be enriched with user specified fields, ex: tags #5029, location data (from exif or manually assigned), default alt text, attribution details, etc

On selecting a file from the media library, the default behavior would be to use the returned entries url as the value for the corresponding file or image field, but we could expose a configuration option to instead return a user configurable field (like an identifier).

**Sample Use Cases**
- A developer already has a large collection of media assets managed in a non-git data store (ex: s3) and would like to improve the UX of allowing editors to select from these assets
- A developer needs the ability to ensure that additional attributes related to the asset are available wherever it is used, ex: license mandated attributions

**Not (initially) in scope**
- Inserting / deleting new media into the collection from the media library UI
- Filtering, displaying or otherwise using additional user provided fields in the media library