added support for kaniko pushretry

[elnoro]

Fixes: #5967
Related: none
Merge before/after: none

Description
Added support for --push-retry flag in Kaniko. Nothing major, just filled in a new constant, a new unit test and changed the api docs.

User facing changes (remove if N/A)

New configuration option pushRetry in the kaniko section.

[tejal29]

@elnoro skaffold team just did a release. Can you please submit a PR for freezing the earlier config and create a new one.
Steps here: https://github.com/GoogleContainerTools/skaffold/blob/master/DEVELOPMENT.md#making-a-config-change

[elnoro]

Hello @tejal29

Thank you! Sure, I can do that - but I thought v20 is the new version? It says "Safe to modify" on L28 in config.go

[codecov]

Codecov Report

Merging #6211 (8067a9d) into master (e02a80d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6211      +/-   ##
==========================================
- Coverage   70.81%   70.80%   -0.01%     
==========================================
  Files         482      482              
  Lines       21874    21876       +2     
==========================================
  Hits        15490    15490              
- Misses       5392     5393       +1     
- Partials      992      993       +1     

Impacted Files	Coverage Δ
pkg/skaffold/schema/latest/v1/config.go	62.16% <ø> (ø)
pkg/skaffold/build/kaniko/args.go	95.55% <100.00%> (+0.10%)	⬆️
pkg/skaffold/util/tar.go	52.87% <0.00%> (-4.60%)	⬇️
pkg/skaffold/docker/parse.go	88.23% <0.00%> (+0.84%)	⬆️

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[tejal29]

yep! I think someone made a PR to freeze config after.