op-mode: T5904: add "show ipv6 route vrf <name> <prefix>" command

We've always had a command to display discrete IPv6 routes/prefixes within the
global VRF. This commit also adds support for a discrete VRF.

vyos@vyos:~$ show ipv6 route vrf <name>
Possible completions:

<Enter>               Execute the current command
<h:h:h:h:h:h:h:h>     Show IPv6 routes of given address or prefix
<h:h:h:h:h:h:h:h/x>