Fix #83: Allow custom response from healthCheck function

[p16]

This PR fixes #83

This PR gives the ability to extend the { status: 'ok' } response using the result of the healthCheck function call.
To allow that we had also to adda new option routeResponseSchemaOpts to the exposeStatusRoute object.

To return something like

{
  status: 'ok',
  extraValue: 'anotherValue'
}

We can configure under-pressure like this

fastify.register(underPressure, {
  ...
  exposeStatusRoute: {
    routeResponseSchemaOpts: {
      extraValue: { type: 'string' },
    }
  },
  healthCheck: async () => {
    return {
        extraValue: 'anotherValue'
    }
  },
}

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm