Add gNMI sample app for XR BGP config

[dasomeom]

Includes two boilerplate apps and seven custom apps to configure BGP for XR data model using gNMI/gNMI: gn-set-xr-ipv4-bgp-cfg-10-ydk.py - set boilerplate gn-set-xr-ipv4-bgp-cfg-40-ydk.py - iBGP IPv4 neighbor grp w/o policy gn-set-xr-ipv4-bgp-cfg-41-ydk.py - iBGP IPv6 neighbor grp w/o policy gn-set-xr-ipv4-bgp-cfg-42-ydk.py - iBGP IPv4 neighbor grp w/ policy gn-set-xr-ipv4-bgp-cfg-43-ydk.py - iBGP IPv6 neighbor group w/ policy gn-set-xr-ipv4-bgp-cfg-44-ydk.py - eBGP IPv4 neighbor grp w/ policy gn-set-xr-ipv4-bgp-cfg-45-ydk.py - eBGP IPv6 neighbor grp w/ policy gn-get-xr-ipv4-bgp-cfg-10-ydk.py - get boilerplate gn-get-xr-ipv4-bgp-cfg-20-ydk.py - get BGP config