Add gNMI sample app for XR SR mapping configuration

[dasomeom]

Includes two boilerplate apps and five custom apps to configure segment routing SID mappings (IPv4 and IPv6) for XR data model using gNMI/gNMI: gn-set-xr-segment-routing-ms-cfg-10-ydk.py - set boilerplate gn-set-xr-segment-routing-ms-cfg-20-ydk.py - IPv4 single mapping gn-set-xr-segment-routing-ms-cfg-21-ydk.py - IPv6 single mapping gn-set-xr-segment-routing-ms-cfg-22-ydk.py - IPv4 multiple mappings gn-set-xr-segment-routing-ms-cfg-23-ydk.py - IPv6 multiple mappings gn-get-xr-segment-routing-ms-cfg-10-ydk.py - get boilerplate gn-get-xr-segment-routing-ms-cfg-20-ydk.py - get segment routing