This removes the health check logic from the ProbeServer and renames the
ProbeServer to UtilityServer that accepts any Rack based app.

The health check and catchall logic are moved into simple Rack middleware
that can be composed by users however they like and be used to preserve
existing health check behavior while transitioning to a more general
purpose utility server.

All and all this pattern will allow users to add whatever functionality
they like to GoodJob's web server by composing Rack apps and using
GoodJob's configuration to pass in users' Rack apps. IE:

```
config.good_job.middleware = Rack::Builder.app do
  use GoodJob::Middleware::MyCustomMiddleware
  use GoodJob::Middleware::PrometheusExporter
  use GoodJob::Middleware::Healthcheck
  run GoodJob::Middleware::CatchAll
end
config.good_job.middleware_port = 7001
```

This could help resolve:

* bensheldon#750
* bensheldon#532

…al-purpose

We decided to leave the original ProbeServer name better sets
expectations. See:

bensheldon#1079 (review)

This also splits out middleware testing into separate specs.

This also helps ensure that the existing behavior and API remain intact.

Since the probe server has the option to use WEBrick as a server
handler, but this library doesn't have WEBrick as a dependency,
we want to throw a warning when WEBrick is configured, but not in the
load path. This will also gracefully fallback to the built in HTTP
server.

As opposed to manipulating the load path.

… rename Rack middleware/app for clarity