Add health check to check a URL

[sleberknight]

This health check will:

• check that a URL returns a 2xx response code
• add a "severity" detail based on the result (uses metrics-healthchecks-severity)
• allow you to provide your own Client, for example to set up custom TLS settings such as key and trust store
• provide an "execution condition" that will conditionally execute the health check, for example only when a service instance is the "leader"

Dropwizard has an HttpHealthCheck but it doesn't let you customize anything, and we specifically need the ability to specify custom key and trust stores for the Client. We also need the conditional execution ability.