Add `skaffold inspect` command to support minimum features required for CC

[tejal29]

In this task

• P0 Support the use case to add a profile with new build env gcb
• P1 Expose if a particular Dockerfile Artifact can be run via Kaniko. Maybe use a Metadata API maybe in BuildMetadata
  Not all DockerArtifacts can be run with kaniko. Some features are not supported in kaniko.
  See notes here https://github.com/tejal29/skaffold/pull/7/files#diff-c3bed1e9b531e926022ffef7a604e2045e899afe03ca1be23de8a3be568c9f8cR398
• P1 Support the use case to add a profile with new build env cluster and artifact type kaniko like this.

  artifacts:
   - image: skaffold-example
     kaniko:
       cache: {}
  cluster:
   namespace: default
  

• P1 Automatically plumb through gcloud auth if image push is true & registry is GCR.
  sample code

[tejal29]

/cc @grmoon and /cc @j-windsor