Autogenerate OpenAPI spec

[imranismail]

A working proof of concept for #4

# /config/config.exs
config :watchtower, Watchtower.Spec,
  router: Watchtower.Router

# /lib/watchtower/spec.ex
defmodule Watchtower.Spec do
  use Parameters.OAS3,
    otp_app: :watchtower,
    info: [
      title: "Watchtower API Spec",
      version: "1.0.0",
    ],
    content_types: [
      api: "application/json"
    ],
    accepts: [
      api: "application/json"
    ]
end