Add digest source 'tag' to use tag without digest

[fengye87]

Related: #3649

Description

A new option skaffold render --digest-source=tag was added, which will render the manifests with built tags. The difference between 'none' and 'tag' is that the former one will keep whatever artifacts are in the original manifests, while the latter will update them with proper tags, without sha256 digests.

This could be very helpful when rendering one-for-all release manifests in multi-arch deployment scenarios, although extra cares should be taken in this case to not move any released tags.

User facing changes (remove if N/A)

User gets a new option skaffold render --digest-source=tag

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[codecov]

Codecov Report

Merging #5436 (5b75087) into master (cac0c53) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5436      +/-   ##
==========================================
+ Coverage   71.39%   71.42%   +0.02%     
==========================================
  Files         397      397              
  Lines       14540    14553      +13     
==========================================
+ Hits        10381    10394      +13     
- Misses       3388     3389       +1     
+ Partials      771      770       -1     

Impacted Files	Coverage Δ
pkg/skaffold/runner/runner.go	0.00% <ø> (ø)
cmd/skaffold/app/cmd/render.go	46.42% <100.00%> (ø)
pkg/skaffold/runner/build_deploy.go	69.23% <100.00%> (+0.34%)	⬆️
pkg/skaffold/docker/parse.go	82.16% <0.00%> (-0.81%)	⬇️
pkg/skaffold/util/config.go	77.77% <0.00%> (ø)
pkg/skaffold/instrumentation/types.go	0.00% <0.00%> (ø)
pkg/skaffold/schema/defaults/defaults.go	87.57% <0.00%> (+0.15%)	⬆️
pkg/skaffold/instrumentation/export.go	76.92% <0.00%> (+0.17%)	⬆️
cmd/skaffold/app/cmd/flags.go	87.93% <0.00%> (+0.43%)	⬆️
pkg/skaffold/test/structure/structure.go	87.50% <0.00%> (+0.70%)	⬆️
... and 2 more

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[MarlonGamez]

Edit: @tejal29 clarified this for me offline, ignore

This may be a dumb question but, what is the difference between using 'remote' vs 'tag'? Since they both go down the same code path now

[MarlonGamez]

LGTM, just one nit to update the comment

[fengye87]

@tejal29 I've updated the comment as @MarlonGamez suggested, could you start another kokoro run please?