Add sample apps for OSPFv3 configuration using gNMI/gNMI

[dasomeom]

Includes two boilerplate apps and five custom apps to configure OSPFv3 for XR data model using gNMI/gNMI: gn-set-xr-ipv6-ospfv3-cfg-10-ydk.py - set boilerplate gn-set-xr-ipv6-ospfv3-cfg-20-ydk.py - single-area IPv6 routing gn-set-xr-ipv6-ospfv3-cfg-30-ydk.py - IPv6 ABR gn-set-xr-ipv6-ospfv3-cfg-32-ydk.py - IPv6 ABR with stub area gn-set-xr-ipv6-ospfv3-cfg-34-ydk.py - IPv6 ABR with totally stub area gn-get-xr-ipv6-ospfv3-cfg-10-ydk.py - get boilerplate gn-get-xr-ipv6-ospfv3-cfg-20-ydk.py - get OSPFv3 config