Support secrets from files

[sbuettner]

Is your feature request related to a problem? Please describe.

Customers are looking for solutions to use a file based secrets approach when running the Connectors runtime.

A SecretProvider implementation that allows to read secrets from files would allow users to use external secret management solutions like Vault for Connector secrets.

Describe the solution you'd like

The solution should support k8s secret management via mounted read-only files and should allow to pass several filepaths via environment variables.

Additional context

https://secrets-store-csi-driver.sigs.k8s.io/topics/sync-as-kubernetes-secret.html