feat: Add add_custom_diagrams serializer

[ewuerger]

This PR adds the new add_custom_diagrams serializer. This serializer is configured like that:

    serializer:
        add_custom_diagrams:
          - capella_attr: realization_view
            polarion_id: realization_view
            title: Realization View Diagram

and therefore multiple custom diagrams can be added as custom fields to a work item. Also there is control about the capella_attr and polarion_id now.

[micha91]

This should not be necessary and does not really look clean to me. You could argue that mocking the elk_input instead is fine as it should be tested in context_diagrams, but I would prefer to work with real input here to ensure that we know, if an update of context diagrams changes the rendering of diagrams

[ewuerger]

The width and height calculations depend from the used platform (MacOS, Linux or Windows). They are not important for identifying if the context-diagram changed, the collected input elements are reliable and give developers the freedom of choice for their used platform without broken tests.