Allow to use multiple authentication methods

[Kaliumhexacyanoferrat]

As a developer of a web service, I would like to use multiple authentication methods so my clients can decide which one to use.

Example

var multiAuth = MultiAuthentication.Create()
                                   .Add(ApiKeyAuthentication.Create())
                                   .Add(BasicAuthentication.Create());

Acceptance criteria

• The functionality is implemented in the Authentication module
• The authentication methods are tried in the order they are added to the concern
• If the status code generated by one of the concerns in order is anything else than 401, the response is returned to the client
• There is a meaningful 401 response when none of the authentication methods worked
• The feature is documented on the GenHTTP website
• The feature is covered by acceptance tests