Enable prometheus monitoring on drools-pdp charts

Drools PDP bundles secrets differently than the rest of the policy fwk components. The serviceMonitor spec to enable prometheus scraping, mandates the metrics endpoint authentication to be provisioned via a secret file. Refactoring the current way of managing credentials in drools-pdp is required in order to provision servicemonitor to expose the metrics to a prometheus instance.

Reference:

• https://github.com/prometheus-operator/prometheus-operator/blob/release-0.38/Documentation/api.md#basicauth
• https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.23/#secretkeyselector-v1-core
• https://gerrit.onap.org/r/c/oom/+/127835