Add --kubeconfig flag for out-of-cluster auth

[cartermckinnon]

Is this a bug fix or adding new feature?

Feature

What is this PR about? / Why do we need it?

This adds a command-line flag, --kubeconfig, that allows an alternative to rest.InClusterConfig(). This is useful when running the CSI driver outside of a Pod.

If --kubeconfig is empty, the behavior is unchanged.

What testing is done?

I see the k8s client instantiated successfully:

> CSI_NODE_NAME=ip-192-168-69-200.us-west-2.compute.internal bin/aws-ebs-csi-driver --kubeconfig ~/.kube/config

I0704 00:50:11.709966   21408 main.go:157] "Initializing metadata"
I0704 00:50:11.710049   21408 metadata.go:66] "The AWS_EC2_METADATA_DISABLED environment variable disables access to EC2 IMDS" enabled="true"
E0704 00:50:11.710363   21408 metadata.go:51] "Retrieving IMDS metadata failed, falling back to Kubernetes metadata" err="could not get EC2 instance identity metadata: operation error ec2imds: GetInstanceIdentityDocument, access disabled to EC2 IMDS via client option, or \"AWS_EC2_METADATA_DISABLED\" environment variable"
I0704 00:50:12.535331   21408 metadata.go:55] "Retrieved metadata from Kubernetes"

[ConnorJC3]

/lgtm

[torredil]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: torredil

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[cartermckinnon]

/retest