Support 404 NotFound with gRPC-JSON transcoder when an unknown url comes in

Currently, if.an unknown or mistyped HTTP request url comes in envoy, it will return something like below(will not delivery to upstream gRPC services):

```http
HTTP/1.1 503 Service Unavailable
content-length: 85
content-type: text/plain
date: Mon, 04 May 2020 13:47:45 GMT
server: istio-envoy
x-envoy-upstream-service-time: 86

upstream connect error or disconnect/reset before headers. reset reason: remote reset
```

It is a client error. Often in HTTP services, returning a 404 is rational.

It would be great envoy provides this feature.