Mesh client tools (apply sub command)

[zhao-kun]

TODO:

• [x] Add descriptions for the PR

[zhao-kun]

The implements divide into three main packages

• util package contains a builder:
  • builder builds a visitor to accept visitFunc to visits all mesh spec resource
  • builder support parse spec from FILEs or directory (support iterates sub-directory recursively), STDIN/STDOUT, and URLs
• meshclient package like client-go of Kubernetes, meshclient is a Clientsets for EaseMesh, you can implement CURD of mesh resources through method chain [1]
• apply package implement logic which is dedicated to applying a mesh resource, apply package composites builders and meshclient

[1] https://github.com/zhao-kun/easemesh/blob/mesh-apply/ctl/cmd/client/command/apply/applier.go#L57