Add Model API to get default Entity Reference values

/**
 * Return default values for specified Entity Reference types. Useful when an Entity Reference value has not been
 * specified and we need to find a default value for it (the default value returned will depend on the implementations,
 * some will return default values defined in the XWiki configuration, others will return values taken from the
 * current document reference, etc).
 *
 * @version $Id$
 * @since 2.3M1
 */
@ComponentRole
public interface EntityReferenceValueProvider
{
    /**
     * @param type the entity reference type for which to get the default value
     * @return the default value for the passed type
     */
    String getDefaultValue(EntityType type);
}