Support for mapping columns from to-one related entity in extractor attributes

[andrus]

"Unflattenning" extractor data matrix into a normalized schema has been a pain so far. This task aims at solving a very common problem in this area. The purpose here is to automatically derive a target root entity FK value that is not present in the source extractor. This can be done by matching one of the extractor source columns against values of a target entity related to root target entity. Then the FK would be equal to PK of such related entity.

Here is how this can be mapped:

<attribute>
   <source>SOURCE_COLUMN</source>
   <target>db:to_one_rel.some_column</target>
</attribute>

Based on this an FK of "db:to_one_rel" relationship should be calculated by LinkMove.