✨ Add support for multi-value KUBECONFIG

[joelanford]

This PR adds support in pkg/client/config.GetConfig() and pkg/client/config.GetConfigWithContext() to handle multi-value KUBECONFIG environment variable values, as documented in the docs about kubeconfig.

It also adds unit tests, which test config loading for the kubeconfig flag, environment variable, and default home directory location. In-cluster configurations are not tested because they require files to exist in hard-coded file system locations.

Closes #564

[k8s-ci-robot]

Hi @joelanford. Thanks for your PR.

I'm waiting for a kubernetes-sigs or kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[mengqiy]

/ok-to-test

[joelanford]

/test pull-controller-runtime-test

[joelanford]

/assign @DirectXMan12

[DirectXMan12]

/lgtm
/approve

[DirectXMan12]

please add a note about where this is set

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: DirectXMan12, joelanford

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