Allow specifying a directory for image credential providers json configuration

[dims]

@micahhausler and i were chatting with some folks about this use case where there may be multiple credential providers, each with possibly their own configuration json files. So i wanted to check if it is as easy as it looks...

Some background:

• Various AMI building tools typically inject a json file with a cloud provider in mind and setup the image-credential-provider-config and the binary
  https://github.com/search?q=repo%3Aawslabs%2Famazon-eks-ami+%2Fimage-credential-provider-config%2F&type=code
  https://github.com/search?q=repo%3Akubernetes-sigs%2Fimage-builder%20%2Fimage-credential-provider-config%2F&type=code
• Then someone who is using the AMI wants to use a different registry as well possibly in addition to or replacing the one baked into the AMI.
• replacing is easy, you can just drop the existing config file, but adding another one is harder as entries for both registries will have to be merged into a single json file. And it becomes even harder to update either the old or new entry if needed (in-place).

So why not support more than one json file? just like we already do for binaries (image-credential-provider-bin-dir is a directory).

What type of PR is this?

/kind feature

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

kubelet: the `--image-credential-provider-config` flag previously only accepted an individual file, but can now specify a directory path as well; when a directory is specified, all .json/.yaml/.yml files in the directory are loaded and merged in lexicographical order.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[dims]

/kind important-soon

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[dims]

/priority important-soon
/kind feature

[liggitt]

I think the simple merging you had before would just error in validation if the same name got defined across files, and that's probably a better behavior (indicates an unexpected / not working configuration)

[dims]

I think the simple merging you had before would just error in validation if the same name got defined across files, and that's probably a better behavior (indicates an unexpected / not working configuration)

will do!

[dims]

/test pull-kubernetes-unit-windows-master

[liggitt]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: b5668fc60abc4f88b25ba7f850ec9e087096f17f

[enj]

First pass.

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@dims: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-kubernetes-unit-windows-master	be6807e	link	false	/test pull-kubernetes-unit-windows-master

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[enj]

/lgtm
/approve
/hold cancel

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 3c452a775dfd82195f0904d1eba764b9956ef58f

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dims, enj, liggitt

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Needs approval from an approver in each of these files:

• cmd/kubelet/OWNERS [dims,liggitt]
• pkg/credentialprovider/OWNERS [dims,enj,liggitt]
• pkg/kubelet/OWNERS [dims,liggitt]

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