Add sample apps to configure OC LLDP using gNMI

[dasomeom]

Includes two boilerplate apps and four custom apps to configure LLDP for OpenConfig using gNMI/gNMI: gn-set-oc-lldp-10-ydk.py - set boilerplate gn-set-oc-lldp-20-ydk.py - enable LLDP gn-set-oc-lldp-22-ydk.py - enable LLDP with timer gn-set-oc-lldp-24-ydk.py - enable LLDP with TLV adv gn-get-oc-lldp-10-ydk.py - get boilerplate gn-get-oc-lldp-20-ydk.py - get LLDP