feature: Cluster level HTTP health check port setting

*Description*:
There's currently a configuration option within [config.endpoint.v3.Endpoint.HealthCheckConfig](https://www.envoyproxy.io/docs/envoy/v1.16.0/api-v3/config/endpoint/v3/endpoint_components.proto.html) for overriding the `port_value`.

The issue with this, or rather, the inconvenience, is that this is configurable per-endpoint. Ideally, we'd have the ability to configure this also at the cluster [config.core.v3.HealthCheck](https://www.envoyproxy.io/docs/envoy/v1.16.0/api-v3/config/core/v3/health_check.proto#config-core-v3-healthcheck) level as well, with the current setting operating as an override if there is no cluster level setting set. This would work (assumingly) identical to the `hostname` value inside of the `HealthCheckConfig` today based on it's documentation:

> By default, the host header for L7 health checks is controlled by cluster level configuration (see: host and authority). Setting this to a non-empty value allows overriding the cluster level configuration for a specific endpoint.