add appProtocol to the hub Service schema

[colinlodter]

Bug description

In our specific use case, we were attempting to scrape Prometheus metrics from JupyterHub's hub pod. In our environment, this is done within an Istio Service Mesh. Prometheus attempts to request the /metrics endpoint using HTTP/2 which Tornado does not support and generates error messages like the following:

 HTTP message from 127.0.0.6: Malformed HTTP version in HTTP Request-Line: 'HTTP/2.0' 

How to reproduce

1. Deploy JupyterHub to a kubernetes cluster with Istio enabled and the standard hub Service manifest.
2. Set up a Prometheus ServiceMonitor for the hub Service.
3. Tail istio-proxy or hub container logs.

Expected behaviour

Allow appProtocol to be set on the hub service (i.e. set to http to "downgrade" requests to the hub service) and specify appProtocol as http.

Actual behaviour

hub application generates errors when HTTP/2 requests are received.

[consideRatio]

Fixed by #3534