Ability to register healthchecks for remote web services

[andrus]

We need a way to check that a remote REST service is alive. Unlike say bootique-jdbc, we don't have a registry of all clients, so we can't install health checks automatically. So we'll simply provide a few REST-oriented health check class(es) that the user may register via vanilla health check API from metrics.

class HttpHealthCheck {
    static HttpHealthCheck checkHEAD(WebTarget) {}
    static HttpHealthCheck checkGET(WebTarget) {}
    static HttpHealthCheck checkOPTIONS(WebTarget) {}
    // other types of health checks require more complex interaction with the server 
    // and can be handcoded without using HttpHealthCheck
}