Draft: Implement a proxy recording new references

To safely remove objects from the archive, we need to know about all references to a given object. While swh.graph gives us a convenient interface to answer this question, it will only be accurate to the time when the graph creation export was made.

Therefore, we need a way to record new references created since this export happened. We introduce a new proxy here wrapping all methods creating new references. They are then recorded using the new object_reference_add interface.

This branch is based on !1042 (merged),

Closes #4683 (closed)