Investigate updating app-config' ConfigMap and extra files' ConfigMap and Secret

[gazarenkov]

Investigate the cases and understand / document limitations (what is supported by K8s and what we need to take care in the Operator) for the case when user change ConfigMap or Secret mounted to Backstage Container, i.e. app-config and extra files
Context:
https://kubernetes.io/docs/concepts/configuration/configmap/#mounted-configmaps-are-updated-automatically says it will be refreshed, but not for env variables and "A container using a ConfigMap as a subPath volume mount will not receive ConfigMap updates."

Related to #74

[gazarenkov]

Another aspect of this problem (mostly for document it), as @rm3l mentioned, is that even if the extra-file is changed, Backstage server does not respect it (unlike app-config).

[gazarenkov]

The conclusion is:

• We do not rely on Kubernets re-mounting as it will not work for all the cases anyway
• So, we consider restarting container (manual or triggered by reconciliation)
• Issue for automatic reconsiciliation created and prioritized