[PageBuilder] MediaGalleryBlock does not export assigned images (file data)

[Algorithman]

Is your feature request related to a problem? Please describe.
The MediaGalleryBlock does not export its assigned image files, only the entity ids. This makes importing difficult if you try to move from one instance to another (local database to live database).
So I thought, I make my own block type (a horizontal slider). Would be good training and I can circumvent the limitation of the media gallery block.
Not so fast. There doesn't seem to be a interface/callback on what to export. Yes there is the StoryAssetKind enum, but no handling of enumerations of images/video/audio. It is only usable for properties which hold a single entity id.

Describe the solution you'd like
A better handling of the data which need to be exported. Please support enumerations of entities (or their ids) for import and export.

[muratcakir]

TBD with MC

[stefanmuellerdo]

(intererst in test / pretest from reporter)