Allow opt-out of always adding healthcheck response headers in the HTTP healthcheck filter

[henrymwang]

Title: Allow opt-out of always adding healthcheck response headers in the HTTP healthcheck filter

Description:
The HTTP health check filter adds response headers in certain cases:

• x-envoy-immediate-health-check-fail, for service-mesh usecases
• x-envoy-upstream-healthchecked-cluster

in other cases (non service-mesh Envoy) it may not be useful to always have those headers added. In our case, we also use the healthcheck filter in "no pass through mode" instead of a direct response route since we rely on the status code to change during server draining.

We'd like to add config to opt out of writing those headers in the H/C filter.

We can prepare the patch for this if it's acceptable.

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