Declare salary | Benefit multiple contracts

Task #4859 | 5064.002

ELM declaration for benefits (KTG and UVGZ) again support multiple contracts for the same domain.

Customers with several UVGZ contracts can now carry out the notifications without manual intervention on the benefit declaration setup. 

The various contracts no longer have to be reported individually, but can also be selected together and submitted with an ELM message.