Add gNMI sample apps for XR OSPF config

[dasomeom]

Includes two boilerplate apps and five custom apps to configure OSPF (single- and inter-area routing) for XR data model using gNMI/gNMI: gn-set-xr-ipv4-ospf-cfg-10-ydk.py - set boilerplate gn-set-xr-ipv4-ospf-cfg-20-ydk.py - single-area IPv4 routing gn-set-xr-ipv4-ospf-cfg-30-ydk.py - IPv4 ABR gn-set-xr-ipv4-ospf-cfg-32-ydk.py - IPv4 ABR with stub area gn-set-xr-ipv4-ospf-cfg-34-ydk.py - IPv4 ABR with totally stub area gn-get-xr-ipv4-ospf-cfg-10-ydk.py - get boilerplate gn-get-xr-ipv4-ospf-cfg-20-ydk.py - get OSPF config