Export bibtex as lists

[floriangantner]

Is your feature request related to a problem? Please describe.
The current bibtex export crosswalk is only configured for single mode, not for list modes.

DSpace/dspace/config/spring/api/crosswalks.xml

Line 81 in f768bbe

<property name="crosswalkMode" value="#{T(org.dspace.content.crosswalk.CrosswalkMode).SINGLE}"/>

Describe the solution you'd like
We would like to have some multiple export format for bibtex as lists too.

Describe alternatives or workarounds you've considered

Additional context
Changing to SINGLE_AND_MULTIPLE shows sufficient results, cause the CSLItemDataProvider also has the method implemented
Single liner on configuration + additional tests