Describe the feature:

Now that #27003 has been implemented, some consumers might want to update multiple objects in that exist in different spaces.

Currently, the bulkUpdate API uses a single namespace field for the entire operation, and this field is transparently set by the SpacesSavedObjectsClientWrapper (not by consumers). In addition to this, we can allow consumers to specify a namespace for each object; if that is specified it will be used instead. Note that the default behavior for bulkUpdate should remain the same.

Describe a specific use case for the feature:

The implementation of #56889 would be greatly simplified by this feature.