Move kubernetes client into its own package

[gsquared94]

Description
Moved the kubernetes client to its own package. This is a prereq to #4701

[codecov]

Codecov Report

Merging #4720 into master will not change coverage.
The diff coverage is 56.25%.

@@           Coverage Diff           @@
##           master    #4720   +/-   ##
=======================================
  Coverage   73.42%   73.42%           
=======================================
  Files         342      342           
  Lines       13529    13529           
=======================================
  Hits         9934     9934           
  Misses       2976     2976           
  Partials      619      619           

Impacted Files	Coverage Δ
pkg/skaffold/build/cluster/kaniko.go	0.00% <0.00%> (ø)
pkg/skaffold/deploy/status_check.go	59.06% <0.00%> (ø)
pkg/skaffold/kubernetes/client/client.go	0.00% <ø> (ø)
pkg/webhook/kubernetes/deployment.go	0.00% <0.00%> (ø)
pkg/webhook/kubernetes/service.go	0.00% <0.00%> (ø)
pkg/skaffold/build/cluster/secret.go	41.53% <50.00%> (ø)
pkg/skaffold/deploy/labels.go	51.28% <100.00%> (ø)
pkg/skaffold/kubernetes/owner.go	87.50% <100.00%> (ø)
...affold/kubernetes/portforward/kubectl_forwarder.go	70.71% <100.00%> (ø)
...ffold/kubernetes/portforward/resource_forwarder.go	84.31% <100.00%> (ø)
... and 3 more

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