Add macaroon_secret_key_path config option

[V02460]

Another config option on my quest to a *_path variant for every secret. This time it’s macaroon_secret_key_path.

Slightly modified the tests to accommodate for this option (see 9367d18, 087ec0f), hopefully self-explanatory.

Reading secrets from files has the security advantage of separating the secrets from the config. It also simplifies secrets management in Kubernetes. Also useful to NixOS users.

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[MadLittleMods]

Thanks for continuing down the path @V02460 🐡