kite: Export an HTTP muxer to add more HTTP endpoints.

Kite had only /kite websocket endpoint which the kites communicate through.

This PR adds a new field, HTTPMuxer, to satisfy the need to add more HTTP
handlers, such as /healthcheck, /version endpoints.
What we could do is, remove this interface and expose a single `kite.HandleHTTP` method. So people can add still `http.Handler` compatible implementation and we can still change the underlying implementation in the future.