Create NamespaceURN constant holder

As explified in NETCONF-1018, constants get easily confused, especially when held together with similar constants with a different use – just as we do in XmlNetconfConstants.

NETCONF-1017 creates CapabilityURN as holder class for URNs used in the context of NETCONF Capabilities. Create NamespaceURN as holder class for URNs used in the context of XML namespaces.

This should make use of constants more steamlined and hopefully will help catching day-0 bugs during code review.