Use RelatedField for signing services

closes #9563

The field referencing a signing service on a publication now uses
pulpcore.plugin.serializers.RelatedField, rather than:
rest_framework.serializers.HyperlinkedRelatedField

The latter was causing a full URL to be stored, which in turn causes
various usability issues and is inconsistent with everything else in
Pulp (which use pulp_href only).

Signed-off-by: Joshua Schmid <jaiks@posteo.de>

CHANGES/9563.bugfix

@@ -0,0 +1 @@
+Fixed a bug causing publications to reference any ``AptReleaseSigningService`` via a full URL instead of just a ``pulp_href``.

pulp_deb/app/serializers/publication_serializers.py

@@ -1,6 +1,7 @@
-from rest_framework.serializers import BooleanField, ValidationError, HyperlinkedRelatedField
+from rest_framework.serializers import BooleanField, ValidationError
 from pulpcore.plugin.models import Publication
 from pulpcore.plugin.serializers import (
+    RelatedField,
     DistributionSerializer,
     PublicationSerializer,
     DetailRelatedField,
@@ -34,7 +35,7 @@ class AptPublicationSerializer(PublicationSerializer):
         default=False,
     )
     structured = BooleanField(help_text="Activate structured publishing mode.", default=False)
-    signing_service = HyperlinkedRelatedField(
+    signing_service = RelatedField(
         help_text="Sign Release files with this signing key",
         many=False,
         queryset=AptReleaseSigningService.objects.all(),