feat(kustomize): Template paths

[sberss]

Description

We have been using Skaffold to allow our developers to create ephemeral namespaces to test code changes within. We were partly able to get this to work by creating a pre-deploy hook that templates the namespace manifest via environment variables, however this has a limitation as you cannot manage multiple namespaces due to not being able to template the Kustomize path.

This PR introduces templating to the Kustomize path by calling the ExpandEnvTemplate utility function. An example project illustrating an example of this workflow can be seen here. With this PR you would be able to create and delete multiple namespaces by setting the NAMESPACE environment variable. The following set of commands would work in this repository if this PR was merged:

NAMESPACE=foo skaffold run
NAMESPACE=bar skaffold run
NAMESPACE=foo skaffold delete

[codecov]

Codecov Report

Merging #7049 (80be480) into main (290280e) will decrease coverage by 1.69%.
The diff coverage is 56.96%.

@@            Coverage Diff             @@
##             main    #7049      +/-   ##
==========================================
- Coverage   70.48%   68.78%   -1.70%     
==========================================
  Files         515      554      +39     
  Lines       23150    25874    +2724     
==========================================
+ Hits        16317    17798    +1481     
- Misses       5776     6867    +1091     
- Partials     1057     1209     +152     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/deploy.go	52.00% <ø> (-1.85%)	⬇️
cmd/skaffold/app/cmd/dev.go	84.61% <0.00%> (ø)
cmd/skaffold/app/cmd/render.go	36.66% <0.00%> (-4.72%)	⬇️
cmd/skaffold/skaffold.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/inspect_tests.go	62.50% <14.28%> (-1.14%)	⬇️
cmd/skaffold/app/cmd/lsp.go	28.12% <28.12%> (ø)
cmd/skaffold/app/cmd/fix.go	68.85% <40.00%> (-7.62%)	⬇️
cmd/skaffold/app/cmd/lint.go	42.85% <42.85%> (ø)
cmd/skaffold/app/cmd/find_configs.go	48.88% <50.00%> (+0.24%)	⬆️
cmd/skaffold/app/skaffold.go	76.19% <70.00%> (-8.43%)	⬇️
... and 184 more

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[aaron-prindle]

Sorry for the review delay. Just looked through, LGTM! Thanks for the PR!