Implement libyang library
This task covers adding a libyang library. It has at least provide:
- makefile changes to build a new lib
- unit-tests
- translation utilities for netconf primitives (int, string, bool, etc) to JSON and vice versa
- a base class for translator
- a base class for watcher (a piece of code that exposes a callback that can be called when certain part of netconf tree changes)
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