Support kubectl deploy using manifest from url

[ghost]

Expected behavior

Sometimes we need to deploy using manifests that doesn't exist in the local machine. like someone only provides the manifest in remote repository, snippet or gists. it would be better if skaffold support deploying using manifest from url.

This might look the following:

apiVersion: skaffold/v1beta1
kind: Config
build:
...
deploy:
  kubectl:
  # ------------- from file ------------- #
    manifests:
    - mylocal/manifest/*.yml
  # ------------- from url -------------- #
    urlManifests:
    - https://gist.githubusercontent.com/someone/somesnippet/raw
    - https://raw.githubusercontent.com/GoogleContainerTools/skaffold/master/examples/getting-started/k8s-pod.yaml

and we don't need to execute kubectl apply -f https://gist.githubusercontent.com/someone/somesnippet/raw for manifests that doesn't exist in the local machine.

Actual behavior

Not supported yet

Steps to reproduce the behavior

1. Add URLManifests in KubectlDeploy struct schema
2. Create some function to read manifest from url eg: using http.NewRequest and append it to kubectl.ManifestList

[ghost]

/close fixed #1408

[williamlord]

I think this has regressed after v0.21.0.