Multiple entity types in one export format

[floriangantner]

Is your feature request related to a problem? Please describe.
The current bulk export format does only support one entity per export format.

Describe the solution you'd like
We would like to export some graph or list of entities from multiple entity types. Not all export formats are suitable due to the configuration amouns, our use cases would be:

• Trigger some export of entities (OrgUnit, Person, Project) of some topic/search in cerif-xml to get some little graph
• Generate some mixed list of entities (Product, Publication) in bibliographic / csl format. Citation for Product / Patent entities #429

Describe alternatives or workarounds you've considered

• output of the single export process has to be appended to some previous output. So perhaps work for some wrapper.
• Extension of the ItemExportFormatService on the backend/frontend necessary

Additional context