Bug 36729: Add publisher/distributor number to standard identifier search index

Have written a patch to allow Publisher/Distributor number to be searched using the standard identifier search index.

Testing plan:

1. Start up KTD with Elasticsearch: ktd --es8 up
2. Apply the patch and restart everything (restart_all).
3. Reset the mappings: Administration > Catalog > Search engine configuration (Elasticsearch) > Reset mappings (at the bottom of the page)
4. Reindex: koha-elasticsearch --rebuild -d -b -a kohadev
5. Alternative to steps 3 and 4: reset_all
6. Update the visibility for 028a in the default framework so that it is visible in the Editor.
7. Add a new record using the default framework and put a term in 028a.
8. Using the advanced search "Standard number" Field, search for the term you put in step 7.

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

admin/searchengine/elasticsearch/mappings.yaml

@@ -2067,6 +2067,10 @@ biblios:
       marc_type: marc21
       sort: 1
       suggestible: ''
+    - facet: ''
+      marc_field: 028a
+      marc_type: marc21
+      sort: 1
     - facet: ''
       marc_field: 010az
       marc_type: unimarc