[regrep-comment] add isValid attribute for ExternalLink

In ebXML reg/rep RIM V2.1 Chapter 7.10:

Since ebXML reg/rep is required to check the validity
of HTTP URIs on submittal is may also be desirable to
have a status filed that allows a registry
implementation to update the validity status (e.g.
up/down, reachable/not reachable) of an ExternalLink.
In conjunction with this it might also be
desirable to have a lastReachable Date/Time field that
indicates when the registry was last able to reach the URI.
[A registry implementation could have a polling agent
which 'walks' through the ExternalLink registry objects
testing each HTTP URI and marking it's status.
Imagine using this in conjunction with the V3
EventNotification feature to also let 'interested
[subscribed] parties' know about a link's status!]

   John Silva
   Email: 
   Contributor to ebxmlrr.sourceforge.net