Support `HEAD` requests on routes

The main use case for this is so that things like tests can use a `HEAD` request to check for proof of life. The recent incident which brought this to our attention was the `/client/request-new-access-token` endpoint. A user was having 404 trouble because they were making a request like `/v1/client/request-new-access-token`. When I alerted them of the pattern, they did a `HEAD` request to `/client/request-new-access-token` to check but still got the 404. In this case, the token endpoints only handle `POST` requests and so there was no matching route for that `HEAD` request.