Add gNMI sample app for XR VRF configuration

[dasomeom]

Includes two boilerplate apps and three custom apps to configure VRFs for XR data model using gNMI/gNMI: gn-set-xr-infra-rsi-cfg-10-ydk.py - set boilerplate gn-set-xr-infra-rsi-cfg-20-ydk.py - IPv4 VRF gn-set-xr-infra-rsi-cfg-21-ydk.py - IPv6 VRF gn-get-xr-infra-rsi-cfg-10-ydk.py - get boilerplate gn-get-xr-infra-rsi-cfg-20-ydk.py - get VRF