Do not close health check connections when draining

*Description*:
When Envoy is draining (via `/healthcheck/fail`) the [HttpConnectionManager will send a GOAWAY](https://github.com/envoyproxy/envoy/blob/main/source/common/http/conn_manager_impl.cc#L1434-L1445) or [Connection:close](https://github.com/envoyproxy/envoy/blob/main/source/common/http/conn_manager_impl.cc#L1476-L1485) on both application and health check connections – even if the health check requests are setting `Connection: keep-alive`.

In an active health checking setup, this causes unnecessary connection churn for health check connections – clients will continue to send health check requests, but will need to open a new connection each time.

This is problematic for servers that (a) receive a lot of health checks (b) use mTLS even for health check requests.

I would expect that if the [health check filter](https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/filters/http/health_check/v3/health_check.proto#envoy-v3-api-msg-extensions-filters-http-health-check-v3-healthcheck) is configured, Envoy would keep these connections alive.

*Repro steps*:
* Configure Envoy as below
* Drain Envoy with `healthcheck/fail`
* Send HTTP/1 health check request with `Connection: keep-alive`
* Observe 503 response with `Connection: close` header

*Config*:
* HttpConnectionManager
* HTTP Health Check filter

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If this behaviour seems reasonable to folks, would love to hear how to approach adding this to Envoy.