server/app/interfaces/repository.py: Implement missing /shells/asset-information/thumbnail endpoint #493

Current Situation

Currently, server endpoint /asset-information/thumbnail, methods=["GET", "PUT", "DELETE"], endpoint=self.not_implemented is not implemented.

According to the AAS Repository Service OpenAPI Specification, the endpoint should return, replace or delete the thumbnail file of a selected AAS.

[...]
responses:
  '200':
    description: The thumbnail of the Asset Information.
    content:
      application/octet-stream:
        schema:
          type: string
          format: binary
[...]

This issue specifies #439.

Proposed Change

We should evaluate and, if feasible, implement GET, PUT and DELETE for /shells/asset-information/thumbnail according to the specification.