Bug 42016: Add identifier-other search field for authorities (MARC 21)

MARC 21 authority records often have 024 $a (Other Standard
Identifier) - https://www.loc.gov/marc/authority/ad024.html,
but the field 024 is missing from ES search fields and the record
cannot be found with these identifiers.

Test plan
=========
1. Have a ktd instance with Elasticsearch.
2. Add a 024 field to an authority record, i.e. for Alatriste, Gustavo:
   024 7# $a 86971927 $7 viaf
3. In Authorities, make an 'Entire record' search for the added
   identifier. You should get no results.
4. Apply the patch, reindex with:
   sudo koha-elasticsearch --rebuild -r -d -a kohadev
5. Repeat p. 3. You should now be able to find the record.

Sponsored-by: Pontificia Università di San Tommaso d'Aquino (Angelicum)
Signed-off-by: Roman Dolny <roman.dolny@jezuici.pl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>

admin/searchengine/elasticsearch/mappings.yaml

@@ -943,6 +943,18 @@ authorities:
     opac: 1
     staff_client: 1
     type: ''
+  identifier-other:
+    label: identifier-other
+    mandatory: ~
+    mappings:
+    - facet: ''
+      marc_field: 024a
+      marc_type: marc21
+      sort: 1
+      suggestible: ''
+    opac: 1
+    staff_client: 1
+    type: ''
   lc-card-number:
     label: lc-card-number
     mandatory: ~