Add MSC3861 config options admin_token_path and client_secret_path (#18004)

Another PR on my quest to a `*_path` variant for every secret. Adds two
config options `admin_token_path` and `client_secret_path` to the
experimental config under `experimental_features.msc3861`. Also includes
tests.

I tried to be a good citizen here by following `attrs` conventions and
not rewriting the corresponding non-path variants in the class, but
instead adding methods to retrieve the value.

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.