Bug 31115: Add additional field filtering for invoice search

Bug 31115: Add additional field filtering for invoice search

This patch adds support for filtering invoice searches on additional
fields. To test:

1. Generate additional fields for invoices
2. Have invoices with additional fields
3. Use invoice searching and play with filtering by additional fields.
=> SUCCESS: It works!
4. Sign off :-D

Sponsored-by: The Research University in the Helmholtz Association (KIT)
Signed-off-by: Michaela Sieber <michaela.sieber@kit.edu>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>