This is library allows us to add rack middleware to proxy requests.

This class can be loaded as Rack middleware and will proxy any requests
with the path prefix of `/assets/static` to backend specified. This is
useful as it enables Static to use relative paths and the dependent
frontend application can forward those requests. This allows us to
switch Static to use relative path and allow us to have development
  environment and not run Static locally (i.e. forward request to
  production version of Static).

This is library allows us to resolve the URL for Static.

This adds a railtie to initialise the proxy middleware with backend set
to the URL resolved by Plek for Static. This allows frontend apps to
enable the proxying behavior in development by simply setting the env
var GOVUK_PROXY_STATIC_ENABLED to "true".