Init Helper for msgpack_obj

[Athishpranav2003]

For complex datatypes such as String, Map, Array and Bin since they have same structure it makes sense to have a common init function to make it easy for dev purposes.

[Athishpranav2003]

@redboltz What about the release cycle for the same?

[redboltz]

Released https://github.com/msgpack/msgpack-c/releases/tag/c-6.1.0