appservice registration yaml: allow `{as,hs}_token` to be specified as `{as,hs_token_path}` instead

Is your feature request related to a problem? Please describe.

This feature is useful for static, secure, repeatable deployments of Matrix servers and appservices. In my exact scenario, without getting into too many details (since they probably abstract over other deployment tools/styles), the visibility/access to the appservice configuration is somewhat necessarily different than the visibility/access to things that need to remain encrypted end-to-end, like the as_token and hs_token.

Luckily, most softwares tend to accept these types of tokens via a path reference or an environment variable, or otherwise have some type of {re,in}direction that allows a relatively elegant or native solution to this layering bit.

Describe the solution you'd like

A revision to the appservice registration yaml file to allow specifying either as_token/as_token_path and either hs_token/hs_token_path.

It might be worth specifying that the file pointed to by *_token_path will be stripped of leading and trailing whitespaces on load, before use.

Describe alternatives you've considered

In this specific situation, there's not a lot of better choices. We can do some creative things like write a service startup script that edits the config files at the very last instance, but this is fairly inelegant, and again, I think this is a fairly common pattern among software for similar types of reasons.

Additional context

None; thank you for working on Matrix and/or reading/considering this!

Start by locating the appservice registration YAML definition in this specification repository and read how the existing as_token and hs_token fields are described. Update the specification to allow the corresponding as_token_path and hs_token_path alternatives, including the requested whitespace-stripping behavior. Done means the accepted alternatives and precedence are unambiguous in the registration format.