Closes #15131: Dynamic queryset annotations for REST API endpoints (#15152)

* Introduce RelatedObjectCountField

* Introduce get_annotations_for_serializer() and enable dynamic annotations

* Add RelatedObjectCountFields to serializers; remove static annotations from querysets

* Remove annotations cleanup logic from BriefModeMixin

* Annotate type for RelatedObjectCountField

* Remove redundant field on TagSerializer

* Add missing reverse relationship for power feeds to rack

* Refactor RelatedObjectCountField to take a single relationship name

netbox/utilities/api.py

@@ -11,10 +11,13 @@ from rest_framework import status
 from rest_framework.serializers import Serializer
 from rest_framework.utils import formatting
 
+from netbox.api.fields import RelatedObjectCountField
 from netbox.api.exceptions import GraphQLTypeNotFound, SerializerNotFound
+from utilities.utils import count_related
 from .utils import dynamic_import
 
 __all__ = (
+    'get_annotations_for_serializer',
     'get_graphql_type_for_model',
     'get_prefetches_for_serializer',
     'get_serializer_for_model',
@@ -131,6 +134,26 @@ def get_prefetches_for_serializer(serializer_class, fields_to_include=None):
     return prefetch_fields
 
 
+def get_annotations_for_serializer(serializer_class, fields_to_include=None):
+    """
+    Return a mapping of field names to annotations to be applied to the queryset for a serializer.
+    """
+    annotations = {}
+
+    # If specific fields are not specified, default to all
+    if not fields_to_include:
+        fields_to_include = serializer_class.Meta.fields
+
+    model = serializer_class.Meta.model
+
+    for field_name, field in serializer_class._declared_fields.items():
+        if field_name in fields_to_include and type(field) is RelatedObjectCountField:
+            related_field = model._meta.get_field(field.relation).field
+            annotations[field_name] = count_related(related_field.model, related_field.name)
+
+    return annotations
+
+
 def rest_api_server_error(request, *args, **kwargs):
     """
     Handle exceptions and return a useful error message for REST API requests.