fix: do not require a 200 from the auth endpoint

[achingbrain]

Some endpoints may require other fields to be passed to the auth endpoint which may result in a non-200 response.

Others may send a different success code (such as 204) so be less strict about the response code, as long as they've sent us the Authentication-Info header.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

[achingbrain]

This came up when trying to integrate with p2p-forge which currently does auth on an endpoint that requires additional arguments and doesn't publish an alternative under /.well-known/libp2p/protocols.

The 200 vs 204 (etc) use case is probably worth handling though.

[MarcoPolo]

Should this check for 2xx instead?

[achingbrain]

That would be slightly more flexible but it wouldn't solve the problem with p2p-forge as it's returning a 400, albeit with a valid Authentication-Info header.

[achingbrain]

Closing in favour of #45