Allow a global timeoutPolicy.response setting

Consider a global override for the envoy <> upstream timeout HTTPProxy.timeoutPolicy.response so it can be configured once for a Contour instance, and not having to specify it on each route. When left alone, the default is set to 15s per Envoy’s docs, so we should make this value configurable. https://github.com/projectcontour/contour/blob/main/site/docs/v1.0.0/httpproxy.md#response-timeout

Consider a similar request for the timeoutPolicy.idle field as well

We also have the request-timeout https://projectcontour.io/docs/main/configuration/#timeout-configuration that controls timeouts for downstream requests.

Start by reading the HTTPProxy timeoutPolicy documentation and the timeout configuration documentation linked in the issue. Determine how a Contour-wide response timeout should interact with per-route response and idle settings and with request-timeout; done should include an agreed configuration behavior for all three cases.

Written by the indexing model from the issue text.