appservice: Add {as,hs}_token_path config options

[V02460]

Adds the config options as_token_path and hs_token_path to the application service configuration. A gratis unit test is part of the deal!

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[sandhose]

I get the idea, but one thing is that the appservice registration files are supposed to be standard, defined by the spec: https://spec.matrix.org/v1.13/application-service-api/

Will raise that to the rest of the team, but I feel like this change will likely have to go through a spec change

[sandhose]

So technically, this isn't allowed by the spec, but we could support this as a synapse 'extension'. Or you should open an MSC to extend the AS spec and support specifying a path for those. What do you think @V02460?

[CLAassistant]

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