Fix port forwarding on Windows

[gsquared94]

Fixes: #3679
Description

On Windows OS, kubectl portforward command launches child processes that do not get cleaned up on the dev loop rerun. This way open ports keep getting locked up.

This PR introduces kubectl.Cmd which embeds exec.Cmd. The Start and Run methods are overridden for Windows OS (using build tags) to wrap around a job object and the job object handle is closed on cancellation to ensure all processes get killed. For non windows OSes it behaves identical to exec.Cmd.

[codecov]

Codecov Report

Merging #4373 into master will decrease coverage by 0.02%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #4373      +/-   ##
==========================================
- Coverage   71.72%   71.69%   -0.03%     
==========================================
  Files         324      325       +1     
  Lines       12500    12554      +54     
==========================================
+ Hits         8966     9001      +35     
- Misses       2966     2977      +11     
- Partials      568      576       +8     

Impacted Files	Coverage Δ
...affold/kubernetes/portforward/kubectl_forwarder.go	60.97% <75.00%> (ø)
pkg/skaffold/kubectl/cli.go	100.00% <100.00%> (ø)
pkg/skaffold/kubectl/exec.go	100.00% <100.00%> (ø)
pkg/skaffold/deploy/util.go	60.97% <0.00%> (-2.92%)	⬇️
cmd/skaffold/app/cmd/deploy.go	66.66% <0.00%> (-1.91%)	⬇️
pkg/skaffold/deploy/kustomize.go	77.05% <0.00%> (-0.66%)	⬇️
pkg/skaffold/config/util.go	70.71% <0.00%> (-0.55%)	⬇️
pkg/skaffold/debug/debug.go	45.16% <0.00%> (ø)
pkg/skaffold/server/server.go	48.88% <0.00%> (ø)
pkg/skaffold/config/options.go	100.00% <0.00%> (ø)
... and 4 more

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