[FEATURE] Support custom one-to-many relations inside fixtures

[nmaier95]

Summary: Implement support to model custom relations of one-to-many relations inside fixture yamls by adding a new config field inside entitySettings for each table called parentRelationColumnName. When set, the data-array structure passed to the core DataHandler receives the relations like expected. The field needs to be set on a child record with the name of the parent record column name of the relation as its value.

Resolves #619