Allow fetching resource metadata from the backend

[reckart]

Is your feature request related to a problem? Please describe.
Annotation editors can load resources such as images or other media from a document from the backend. Sometimes, it would be useful to be able to read just metadata about the object (e.g. its dimensions) instead of the entire object.

Describe the solution you'd like
Add a parameter do the resource endpoint by which the browser can request only the metadata about the object instead of the entire object.