Support using in-cluster kubeconfig

[Duke0404]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Adds support for using in-cluster Kubernetes configuration as fallback in case neither path to Kubernetes configuration nor location of master is provided.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Allows the user to use in-cluster kubernetes configuration while self-hosting cluster-autoscaler as a pod within their cluster.

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

/cc @aleksandra-malinowska @yaroslava-serdiuk

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Duke0404 / name: Devansh Das (5420a95, 1dba38e, 23e8371, 351748c, ee6d4f9, ed38cbb)

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @Duke0404. Thanks for your PR.

I'm waiting for a 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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[yaroslava-serdiuk]

/ok-to-test

[yaroslava-serdiuk]

Please, fix gofmt:

Please run hack/update-gofmt.sh to fix the following files:
./cluster-autoscaler/utils/kubernetes/client.go

[aleksandra-malinowska]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aleksandra-malinowska, Duke0404

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• cluster-autoscaler/OWNERS [aleksandra-malinowska]

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