Bug 36953: Add 678 to authority index (MARC21)

Bug 36953: Add 678 to authority index (MARC21)

This enhancement adds field 678 to the default MARC21 authority index mappings.

To test:
1. Import the attached .mrc file
2. Go to Authorities
3. Search "Entire record" for innu
   --> No results
4. Go to the OPAC > Authority search
5. Search "in the complete record" for innu
   --> No results
6. Apply patch
7. Reindex authorities (in KTD shell:
   koha-elasticsearch --rebuild -a -v -r kohadev)
8. Redo step 3
   --> 1 result
9. Redo steps 4-5
   --> 1 result

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>