feat: handle path list for kubeconfig env var

[ndbaker1]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR implement simple parsing of $KUBECONFIG as a path list, similar to how it is done in client-go:

• see documented loading order
• see client-go implementation

this allows users to do things like pickup a directory-scoped kubeconfig as well as the home directory fallback using the following config:

KUBECONFIG=.kube/config:~/.kube/config

Testing

> go test ./klient/conf/... -v
I0719 21:52:56.402946  827913 main_test.go:71] file created successfully/home/nbakerd/test/.kube/config
=== RUN   TestResolveKubeConfigFileFlag
--- PASS: TestResolveKubeConfigFileFlag (0.00s)
=== RUN   TestResolveKubeConfigFileEnv
=== RUN   TestResolveKubeConfigFileEnv/WithEnvEmpty
=== RUN   TestResolveKubeConfigFileEnv/WithEnvPath
=== RUN   TestResolveKubeConfigFileEnv/WithEnvPathListAllExist
=== RUN   TestResolveKubeConfigFileEnv/WithEnvPathListFirstExists
=== RUN   TestResolveKubeConfigFileEnv/WithEnvPathListLastExists
=== RUN   TestResolveKubeConfigFileEnv/WithEnvPathListNoneExist
--- PASS: TestResolveKubeConfigFileEnv (0.00s)
    --- PASS: TestResolveKubeConfigFileEnv/WithEnvEmpty (0.00s)
    --- PASS: TestResolveKubeConfigFileEnv/WithEnvPath (0.00s)
    --- PASS: TestResolveKubeConfigFileEnv/WithEnvPathListAllExist (0.00s)
    --- PASS: TestResolveKubeConfigFileEnv/WithEnvPathListFirstExists (0.00s)
    --- PASS: TestResolveKubeConfigFileEnv/WithEnvPathListLastExists (0.00s)
    --- PASS: TestResolveKubeConfigFileEnv/WithEnvPathListNoneExist (0.00s)
=== RUN   TestNew
--- PASS: TestNew (0.00s)
=== RUN   TestNewWithContextName
--- PASS: TestNewWithContextName (0.00s)
PASS
ok  	sigs.k8s.io/e2e-framework/klient/conf	0.005s

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

the KUBECONFIG env var can now be provided as a list of paths

Additional documentation e.g., Usage docs, etc.:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ndbaker1
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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @ndbaker1. Thanks for your PR.

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[vladimirvivien]

Hi @ndbaker1 thanks for this PR.
Please add the followings

• Tests (with different scenarios) to catch regressions
• Documentation showing how this would work.

Thanks.

[ndbaker1]

thanks for looking @vladimirvivien! i added unit tests with my latest revision and improved the PR overview.
please let me know what else you recommend in terms of documentation!

[vladimirvivien]

@ndbaker1 this looks good. Please add an example markdown doc.