Enhancement: Remove use of downward API from manifest

[sigalsax]

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Current solution: Customer supplies these values using Downward API for MY_POD_NAME and MY_POD_NAMESPACE env vars in Secrets Provider manifest.

Enhanced solution: Get pod's namespace from /var/run/secrets/kubernetes.io/serviceaccount/namespace file inside the container. Get pod's name from HOSTNAME env var (as documented in K8s docs)

Motivation: better UX, no need to maintain another env vars.

Limitation: HOSTNAME is the pod's name only if customer didn't supply a hostname explicitly.

Solution: Use HOSTNAME if MY_POD_NAME is not supplied