feat(CLI): Add ValidateFileOrEnv function in kubeconfig package

[ewollesen]

ValidateFileOrEnv validates a given kubeconfig filename, or, if and only if that filename is an empty string, falls back to a kubeconfig filename from the environment, and validates it.

This should be useful for CLI commands that want to find the first specified kubeconfig file that's fit to use.

While this isn't used yet, my next PR will use it. :)

Issue #, if available:

Description of changes:

Testing (if applicable):

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[codecov[bot]]

Codecov Report

Merging #3309 (7553a55) into main (629d786) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3309      +/-   ##
==========================================
+ Coverage   64.41%   64.44%   +0.02%     
==========================================
  Files         352      352              
  Lines       27980    28001      +21     
==========================================
+ Hits        18023    18044      +21     
  Misses       8648     8648              
  Partials     1309     1309              

Impacted Files	Coverage Δ
pkg/kubeconfig/kubeconfig.go	95.34% <100.00%> (+4.43%)	:arrow_up:

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[ewollesen]

/approve

[a-cool-train]

/approve /lgtm

[eks-distro-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: a-cool-train, ewollesen

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