feat: populate cluster config from client.authentication.k8s.io/exec …

[ma-ts]

What type of PR is this?

/kind bug

What this PR does / why we need it:

populate ExecCredentialSpec.Cluster.config from the client.authentication.k8s.io/exec extension

Which issue(s) this PR fixes:

Fixes #2395

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Adds cluster config information to the exec credential provider if the client.authentication.k8s.io/exec extension is provided.

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

Recreating PR since #2396 has CLA issues. Credits go to @wkloucek

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ma-ts
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