Support private encoding option from draft-ietf-netconf-udp-notif-03

[pierre-francois]

Support of the private encoding option described in draft-ietf-netconf-udp-notif-03 Use temporary type: 2

• [x] Update parser
• [x] Update metadata struct to provide the option value to the lib user

[ahuangfeng]

Private encoding and custom options are implemented and merged on development branch. Will be released in v0.5.2.