[Assets] Add image thumbnail option to custom image stream endpoint

[markus-moser]

Add an option to define a thumbnail definition to the /pimcore-studio/api/assets/{id}/image/stream/custom endpoint.

We need this for the document editables to create a pendant to this request: /admin/asset/get-image-thumbnail?id=337&thumbnail=portalCarousel

[lukmzig]

@markus-moser I added a new endpoint /pimcore-studio/api/assets/{id}/video/stream/{thumbnailName} so we do not mix custom thumbnail with pre-defined ones