feature: Cluster level HTTP health check port setting

Description: There's currently a configuration option within config.endpoint.v3.Endpoint.HealthCheckConfig 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 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.