Add LibManager for managing k8s API and ksonnet-lib metadata

This change will introduce a lib package that handles the reading,
writing, and serialization of the ksonnet-lib and the kubernete's open
API files.

Prior to this change, metadata was stored in an environment's .metadata
directory. This lead to redundant files where environments share the
same kubernetes API version.

Signed-off-by: Jessica Yuen <im.jessicayuen@gmail.com>