♻️(api) refactor getting versions to expose pagination

Getting versions was not working properly. Some versions returned
were not accessible by the user requesting the list of available
versions.

We refactor the code to make it simpler and let the frontend handle
pagination (load more style).

src/backend/core/api/serializers.py

@@ -343,10 +343,10 @@ class InvitationSerializer(serializers.ModelSerializer):
         return attrs
 
 
-class DocumentVersionSerializer(serializers.Serializer):
-    """Serialize Versions."""
+class VersionFilterSerializer(serializers.Serializer):
+    """Validate version filters applied to the list endpoint."""
 
-    etag = serializers.CharField()
-    is_latest = serializers.BooleanField()
-    last_modified = serializers.DateTimeField()
-    version_id = serializers.CharField()
+    version_id = serializers.CharField(required=False, allow_blank=True)
+    page_size = serializers.IntegerField(
+        required=False, min_value=1, max_value=50, default=20
+    )