Custom page for 500, 501, 502, 503, ... HTTP status

**What would you like to be added**:

When the backend returns Server Error (500) we want to change the response page to hide any security sensitive information.

Similarily for Bad Gateway (502), we want to change the error page to something less technical (and different between implementations): a maintenance page.

Same for 503 (Service Unavailable), and maybe others (404 Not Found, ...).

I don't know how to do this. Maybe `responseRules` that can match on response status and filter with a `ResponseBodyModifier`.

envoy is able to do this with [custom_response_filter](https://www.envoyproxy.io/docs/envoy/v1.34.0/configuration/http/http_filters/custom_response_filter).

**Why this is needed**:

We want to hide technical details to users.

The issue names no repository file or test. Start by reviewing the proposed responseRules and ResponseBodyModifier concepts, then compare the Envoy custom_response_filter reference. Done should define an implementable API for replacing selected backend error pages while hiding technical details.

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