Generated openapi.json should contain a `servers` section with `localhost:8080`

[rob-bygrave]

Like:

  "openapi" : "3.0.1",
  "info" : { 
      ...
  },
  "servers": [
    {
      "url": "localhost:8080",
      "description": "local testing"
    }
  ],

This then means that tooling like IntelliJ can automatically use it for local testing.