Support multiple apps in Plug.SwaggerUI

[zorbash]

Currently the OpenApiSpex.Plug.SwaggerUI does not support multiple URLs and to preview more than a single spec one has to add the plug n times.

The SwaggerUI supports this already docs through the topbar plugin which is loaded in the SwaggerUIStandalonePreset preset open_api_spex uses.

This PR allows setting a paths keyword instead of a path string.

scope "/" do
  pipe_through :browser # Use the default browser stack
  get "/", MyAppWeb.PageController, :index
  get "/swaggerui", OpenApiSpex.Plug.SwaggerUI,
    paths: [
      latest: "/api/openapi",
      legacy: "/legacy_api/openapi",
      other_app: "http://localhost:4001/my_other_app/api/openapi"
    ]
end

Which renders as seen in the screenshot below:

It does not affect routers not using the new paths option.