Add default file dependency handling during exports and imports

As per discussion with Wim - this is BE piece allowing us to export and import default images for js_component entities. This is based on PoC from a year ago, but updated to lates state of things.

Images are base64 decoded on export and set on a property of .yml file. During import the image file is decoded, created (if necessary) on the destination environment and unset from DB version of config entity. Uuid of the encoded File entity is based on hash of it's content, so we can check if identical image already exists.

Closes #3532574