Add gNMI sample apps for XR telemetry config

[dasomeom]

Includes two boilerplate apps and nine custom apps to configure XR telemetry using gNMI/gNMI: gn-set-xr-telemetry-10-ydk.py - set boilerplate gn-set-xr-telemetry-20-ydk.py – tcp dialout single sensor-group/subscription gn-set-xr-telemetry-22-ydk.py – grpc dialout multiple sensor-paths gn-set-xr-telemetry-24-ydk.py – grpc dialout multiple sensor-groups gn-set-xr-telemetry-26-ydk.py – grpc dialout multiple destinations gn-set-xr-telemetry-28-ydk.py – grpc-tls dialout with multiple destination-groups gn-set-xr-telemetry-30-ydk.py - dialin single sensor-group/subscription gn-set-xr-telemetry-32-ydk.py - dialin multiple sensor-paths gn-set-xr-telemetry-34-ydk.py - dialin multiple sensor-groups gn-get-xr-telemetry-10-ydk.py - get boilerplate gn-get-xr-telemetry-20-ydk.py - get telemetry