Bug 35197: Add additional_fields REST API endpoint

Bug 35197: Add additional_fields REST API endpoint

Test plan:
1) Apply patch, restart plack 'koha-plack --restart kohadev'
2) Visit /api/v1/additional_fields?tablename=aqinvoices - Notice its empty
3) Visit /cgi-bin/koha/admin/additional-fields.pl?tablename=aqbasket and add a new additional field
4) Do step 2) again - Notice the newly created additional field is there
5) Visit /cgi-bin/koha/admin/additional-fields.pl?tablename=aqinvoices and add a new additional field for invoices
6) Do step 2) again - Notice both additional fields are there
7) Visit /api/v1/additional_fields?tablename=aqinvoices - Notice only the additional field for aqinvoices is listed

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>