Adding support to dynamically discover metrics configs across all namespaces in the cluster

[ramasai1]

It'd be great if the adapter could dynamically discover metrics configurations across all namespaces in the cluster. This would be helpful in scenarios where application developers can add Prometheus rules for HPA in their Helm chart without needing to touch the adapter configuration.

A similar ask is present in https://github.com/kubernetes-sigs/prometheus-adapter/issues/662, but I've chosen to add support using ConfigMap since the adapter already uses that instead of adding a new CRD.

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ramasai1 Once this PR has been reviewed and has the lgtm label, please assign dgrisonnet for approval. For more information see the Kubernetes Code Review Process.

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[dgrisonnet]

/assign /triage accepted