Add client_secret_path as alternative for client_secret for OIDC config

[Ma27]

That way you don't have to leak your bind password into your config. Useful for e.g. NixOS where config is stored in a world-readable location.

Tested against a live synapse instance with authentik as OIDC provider.

Pull Request Checklist

• Pull request is based on the develop branch
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[clokep]

Looks pretty reasonable; I wish we had a more abstract way of doing this instead of manually each time we want something to be a file.

[Ma27]

Added relevant notes and squashed this and the previous commit together :)

[clokep]

Thanks! Sorry for the back-and-forth here.