Essential improvements to the HTTP API used by https://github.com/matrix-org/mjolnir-widget

[Gnuxie]

These are improvements that have to be coordinated with the widget, but are also needed to be implemented before any other changes are made.

• [ ] Put the openID token into the authorization header as a Bearer token and validate the request with middleware so we're not bodging it in each api endpoint.
• [ ] Declaratively specify the format of request bodies for validation using some middleware for express.
• [ ] Namespace the endpoints, we are almost certainly going to have a web api for controlling a specific mjolnir in future too and these shouldn't clash.
• [ ] Update the test client used for the appservice integration tests accordingly
• [ ] Can resolve openID tokens from different homeservers than just the one the appservice is deployed on https://github.com/matrix-org/mjolnir/pull/364/files#r1023075345