Add config option redis.password_path

[V02460]

Adds the option to load the Redis password from a file, instead of giving it in the config directly. The code is similar to how it’s done for registration_shared_secret_path. I changed the example in the documentation to represent the best practice regarding the handling of secrets.

Reading secrets from files has the security advantage of separating the secrets from the config. It also simplifies secrets management in Kubernetes.

The second commit adds test cases for secret files existing and missing.

Pull Request Checklist

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