Add gNMI sample app for XR keychain config

[dasomeom]

Includes two boilerplate apps and four custom apps to configure keychain for XR data model using gNMI/gNMI: gn-set-xr-lib-keychain-cfg-10-ydk.py - set boilerplate gn-set-xr-lib-keychain-cfg-20-ydk.py - one AES-128-GCM key gn-set-xr-lib-keychain-cfg-22-ydk.py - one AES-128-GCM key gn-set-xr-lib-keychain-cfg-24-ydk.py - two rolling keys gn-get-xr-lib-keychain-cfg-10-ydk.py - get boilerplate gn-get-xr-lib-keychain-cfg-20-ydk.py - get boilerplate