Add gNMI sample app for XR MPLS IP TTL configuration

[dasomeom]

Includes two boilerplate apps and two custom apps to configure global MPLS attributes (MPLS LSD) for XR data model using gNMI/gNMI: gn-set-xr-mpls-lsd-cfg-10-ydk.py - set boilerplate gn-set-xr-mpls-lsd-cfg-20-ydk.py - disable for all packets gn-get-xr-mpls-lsd-cfg-10-ydk.py - get boilerplate gn-get-xr-mpls-lsd-cfg-10-ydk.py - get global MPLS attributes