Support restana server factory

In this release we extend the gateway configuration related to restana, basically we are allowing to overwrite the restana instantiation inside the gateway by providing a factory method.

const gateway = require('fast-gateway')
const restana = require('restana')
const serverInstance = restana({})

const server = gateway({
  restana: () => serverInstance,
  //...
})

The goal is to allow more complex restana initialisations such as the Elastic APM support.