Enables PATCHing some attributes in attribute_set of a dataset

For now the payload is wrapped in `data` like so:

```json
{
  "data": {
     "attribute_set": [{
       "pk": 1, 
       "description": "updated description" 
     }]
  }
}
```

which skips (more or less) `rest_framework` validation.

geonode/layers/api/serializers.py

@@ -17,7 +17,7 @@
 #
 #########################################################################
 from rest_framework import serializers
-
+from rest_framework.exceptions import ValidationError, ParseError
 from urllib.parse import urlparse
 
 from django.conf import settings
@@ -195,6 +195,37 @@ class Meta:
 
     featureinfo_custom_template = FeatureInfoTemplateField()
 
+    def update(self, instance, validated_data):
+        super().update(instance, validated_data)
+
+        # Handle updates to attribute_set
+        allowed_fields = ["pk", "description", "attribute_label", "visible", "display_order"]
+        if "blob" in validated_data and "attribute_set" in validated_data["blob"]:
+            attributes = validated_data["blob"]["attribute_set"]
+
+            for attribute in attributes:
+                for field, _ in attribute.items():
+                    if field not in allowed_fields:
+                        raise ValidationError(
+                            f"{field} is not one of the fields that could be edited directly. \
+                                            Only {str(allowed_fields)} are allowed"
+                        )
+
+            for attribute in attributes:
+                try:
+                    if "pk" in attribute:
+                        attribute_instance = Attribute.objects.get(pk=attribute["pk"], dataset=instance)
+                        for field, value in attribute.items():
+                            setattr(attribute_instance, field, value)
+                        attribute_instance.save()
+                    else:
+                        raise Exception("Primary key of the attribute to be patched not specified")
+                except Exception as e:
+                    logger.error(e)
+                    raise ParseError(str(e))
+
+        return instance
+
 
 class DatasetListSerializer(DatasetSerializer):
     class Meta(DatasetSerializer.Meta):