Add `mount` method

[i8ramin]

Allows you to mount routes to different proxied backends (supports RegExp)

Example:

 import { mount, MountInfo, proxy, middleware, pipeline } from '@fly/cdn'

 const routes: MountInfo[] = [
  ['/blog', proxy('https://medium.com/blog')],
  ['/docs', proxy('https://docs.example.com/docs')],
  [/^\/(?:[a-z]{2}(-[A-Z]{2})?\/)?app/, proxy('https://app.example.com')] // ie, /de-DE/products
 ]

 const p = pipeline(middleware.httpsUpgrader, middleware.autoWebp)
 const routerApp = p(mount(routes))

 fly.http.respondWith(routerApp)

[i8ramin]

Looks like the aws backend tests are failing for some reason. Unrelated to this change. https://dev.azure.com/flydotio/fly/_build/results?buildId=941

[i8ramin]

@KittyBot, @mrkurt any ideas?

[i8ramin]

Btw, this has happened before in another PR -- https://github.com/superfly/edge/pull/45#issuecomment-462967729