vmauth: add ability to provide external configuration via Vault Sidecar

[AndrewChubatiuk]

Currently VMAuth supports configuration management via:

• config-reloader
• externally managed kubernetes secrets Vault Sidecar doesn't rely on k8s secrets and renders templates inside pod. Need to disable config-reloader and configuration management via secrets if path to vmauth configuration is set

[f41gh7]

Feature included to the v0.49.0 release