Add support for changing the minimum healthy status code

[jeffmercerVG]

I have a use case where I am running Envoy and OPA in front of a SpringBoot app with the Lambda Web Adapter. While OPA is starting up, Envoy will return a 4xx error code. The Lambda Web Adapter sees the 4xx as less than 500 and thinks the app has successfully started up. I would like an option to change the definition of unhealthy in the LWA
Note: I have a PR incoming for this.

[bnusunny]

Thanks for this contribution. This is indeed necessary.