Import: lookup entity reference fields using multiple columns of an import

[sleidig]

When importing data that includes a link to existing records, I may need multiple columns to accurately match the right record to be linked in the import, e.g. using both firstname and lastname fields (if these are stored separately).

Objective
We currently are able to import "entity" dataType fields by looking up one column from the import and selecting an existing entity matching that value. Extend this to use a combination of multiple fields that all (?) need to match, for more accurately linking.

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Implementation Steps:

• extend the EntityDatatype.importMapFunction to support this (and write unit tests for it first)
• define an interface what "config" data is passed into the importMapFunction as "additional" (as this now has to contain more than just the ID of one matching field)
• extend the EntityImportConfigComponent to offer a UI for configuring multiple fields