Bug 22474: Clone field config before adding fields to it

Bug 22474: Clone field config before adding fields to it

Test plain:

1. Reindex both authorities and biblios with the -d flag to recreate the index, e.g. "perl misc/search_tools/rebuild_elastic_search.pl -v -d"
2. Check that authorities only contains authority-specific fields in Elasticsearch e.g. by fetching  http://localhost:9200/koha_dev_authorities/_mappings
3. Reindex both without the -d flag and verify it works, e.g. "perl misc/search_tools/rebuild_elastic_search.pl -v"

Signed-off-by: Björn Nylén <bjorn.nylen@ub.lu.se>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>