Use response_headers_to_add for Envoy-generated error responses

[ascheglov]

On events like a request timeout, Envoy sends a locally generated response (the sendLocalReply function). But it doesn't use the response_headers_to_add config option. Which could be useful when response_headers_to_add has some diagnostic information (e.g. the %REQ(x-request-id)% which was added in #8683)

We can fix this by adding finalizeResponseHeaders calls before sendLocalReply calls, in functions like onUpstreamAbort.

[lizan]

Wondering whether the local reply mapper can address this, basically it will allow you to customize all Envoy-generated error responses. See #8921

[Keramblock]

Is there any news about it? We need that too.