Add gNMI sample apps for time zone config

[dasomeom]

Includes two boilerplate apps and six custom apps to configure device time zone for XR data model using gNMI/gNMI: gn-set-xr-infra-infra-clock-linux-cfg-10-ydk.py - set boilerplate gn-set-xr-infra-infra-clock-linux-cfg-20-ydk.py - PST PST8PDT gn-set-xr-infra-infra-clock-linux-cfg-22-ydk.py - CST PRC gn-set-xr-infra-infra-clock-linux-cfg-24-ydk.py - CET CET gn-set-xr-infra-infra-clock-linux-cfg-26-ydk.py - BRT Brazil/East gn-set-xr-infra-infra-clock-linux-cfg-28-ydk.py - WAT Africa/Douala gn-get-xr-infra-infra-clock-linux-cfg-10-ydk.py - get boilerplate gn-get-xr-infra-infra-clock-linux-cfg-20-ydk.py - get device time zone