Extend RunFn to be able to handle relative source paths

[aodinokov]

It's an attempt to propose some solution for kptdev/kpt#1151 for bind mount type.

this test shows how kustomize build may benefit from that feature - it's possible to pass some additional files inside krm-function and use them there like they were local. In particular helm-inflator in this example takes values overrides in this way.

The security perspective is covered by sandboxing non-absolute paths inside StorageMountRoot which is equal to path in kustomize fn run <path> or kustomization directory in case of kustomize build, but can be manually overriden by setting mount-root parameter. non-absolute path will throw error if mount-root is empty.

[k8s-ci-robot]

Hi @aodinokov. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

@aodinokov: PR needs rebase.

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