Netconf With-defaults Capability

[pheller]

It is desirable to support https://datatracker.ietf.org/doc/html/rfc6243 such that elements within a configuration datastore can be identified as default or non-default without the need for schema awareness. For example:

    <config xmlns:nc2="urn:ietf:params:xml:ns:netconf:default:1.0" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
      <sys:system xmlns:sys="urn:ietf:params:xml:ns:yang:ietf-system">
        <sys:ntp>
          <sys:enabled nc2:default="true">true</sys:enabled>
        </sys:ntp>
      </sys:system>
    </config>

With this supported, subsequent features can be supported, like show configuration | display defaults, and thus show configuration could omit defaults for a more terse display.

[olofhagsand]

feature implemented and merged: e39d18d

[pheller]

Tested NETCONF, RESTCONF, and CLI - looks good to me, thanks!