Allow specifying separate port for health check

I have the following settings of the proxy in kamal.yaml file

proxy:
  app_port: 8080
  response_timeout: 10
  healthcheck:
    interval: 3
    path: /health
    timeout: 3

Usually, the "management" (a.k.a observability - health, metrics, prometheus etc.) endpoints are served on a different port not exposed to outside world, e.g. 8181

Could you please add another property to specify different port for the health check, so we end up with

proxy:
  app_port: 8080
  response_timeout: 10
  healthcheck:
    interval: 3
    path: /health
    port: 8181 # the new shiny property
    timeout: 3