Add support for GND identifiers on Authors

Fixes #8037

Using “Integrated Authority File” rather than the “Universal” from the
ticket, since “Integrated” is the translation the DNB themselves use:
https://www.dnb.de/EN/Professionell/Standardisierung/GND/gnd_node.html

DNB editions generally use DNB identifiers, which seem to be different
from GND ones, so no need to add those for Editions; but GND is used
for Works too, so this should also be added for them when we get those
into code.

Note that there are still some potential Authors that are still using
DNB identifiers, but the DNB’s plan is to transition all persons,
organisations, etc. to use GND ids, so not adding DNB ids here for now
unless it turns out they’re actually needed/desired for something.

Corresponds to Wikidata’s https://www.wikidata.org/wiki/Property:P227
and is part of MusicBrainz’s https://musicbrainz.org/relationship/d94fb61c-fa20-4e3c-a19a-71a949fb2c55

openlibrary/plugins/openlibrary/config/author/identifiers.yml

@@ -17,6 +17,11 @@ identifiers:
     notes: ''
     url: https://isni.org/isni/@@@
     website: https://isni.org/
+-   label: Integrated Authority File (GND)
+    name: gnd
+    notes: ''
+    url: https://d-nb.info/gnd/@@@
+    website: https://gnd.network/
 -   label: IMDb
     name: imdb
     notes: Should be something like nm0393654