Add MSC3861 config options admin_token_path and client_secret_path

[V02460]

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.

It is noteworthy that in this patch each access to the client secret or the admin token will re-read it from file. In the current state a ConfigError will be raised at runtime when the file can not be read anymore. You might prefer another form of error handling than the current behavior.

A big pro of directly using the secret contents from file is that it enables secret rotation without having to restart Synapse.

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