[FEATURE] Support 1:n relations in mappings

Extend the mapping configuration to allow for multiple values in the same field,
to be mapped individually onto 1:n or m:n relations.

Resolves: #52664
Releases: 2.3
Change-Id: I546c8fc9da1a6a0821b52e735f3fa151dfc39c2c
Reviewed-on: https://review.typo3.org/30327
Reviewed-by: Francois Suter
Tested-by: Francois Suter