Make it possible to compute the isEntryReadonly boolean flag dynamically

Use case: on a LDAP server we would like to have some entries that are editable using the regular nuxeo interface, while others are not based of the values of field of the fetched entry.

This can be implemented as a new extension point to the LDAP and SQL directory that allows to plug a class that implements a generic adapter interface that builds a document model instance out of the DocumentModel entry and the Directory instance.

A generic implementation that updates the readonly entry boolean flag based on regexp matching on string valued fields be provided and should cover the basic use case while allowing advanced custom cases by overriding the implementation.