add support for document $self property

[baywet]

OAI 3.2.0 adds a new $self property to the document object.

This property should be a Uri? and serialization/deserialization should be updated to account for the new property.

When serializing to OAI 3.1.0 or lower, the property should be prefixed with x-oai- as an exension.

We will NOT implement the updates to references resolution as part of this issue.