Add commands to list/print json schemas

[dgageot]

$ skaffold schema
List and print json schemas used to validate skaffold.yaml configuration

Available Commands:
  get         Print a given skaffold.yaml's json schema
  list        List skaffold.yaml's json schema versions

$ skaffold schema list
skaffold/v1alpha1
skaffold/v1alpha2
skaffold/v1alpha3
skaffold/v1alpha4
skaffold/v1alpha5
skaffold/v1beta1
skaffold/v1beta2
skaffold/v1beta3
skaffold/v1beta4
skaffold/v1beta5
skaffold/v1beta6
skaffold/v1beta7
skaffold/v1beta8
skaffold/v1beta9
skaffold/v1beta10
skaffold/v1beta11
skaffold/v1beta12
skaffold/v1beta13
skaffold/v1beta14
skaffold/v1beta15
skaffold/v1beta16
skaffold/v1beta17
skaffold/v1
skaffold/v2alpha1

$ skaffold schema get skaffold/v1
{
  "type": "object",
  "anyOf": [
    {
      "$ref": "#/definitions/SkaffoldConfig"
    }
  ],
...
}

Signed-off-by: David Gageot david@gageot.net

[codecov]

Codecov Report

Merging #3355 into master will decrease coverage by 0.01%.
The diff coverage is 78.37%.

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/schema/fs_dummy.go	0% <0%> (ø)
cmd/skaffold/app/cmd/cmd.go	67.44% <100%> (+0.25%)	⬆️
cmd/skaffold/app/cmd/schema/list.go	100% <100%> (ø)
cmd/skaffold/app/cmd/schema/print.go	80% <80%> (ø)
cmd/skaffold/app/cmd/schema.go	80% <80%> (ø)
pkg/skaffold/util/tar.go	52.87% <0%> (-4.6%)	⬇️