Support dynamic App Store auth endpoint discovery

[efibalogh]

Summary

Resolve issues with App Store authentication by implementing dynamic auth endpoint discovery. If the initial auth request fails due to endpoint redirects or changes, the client parses the XML plist unmarshal error for any new authentication endpoint and automatically retries the login.

Ports the fix from ipatool (credits to @b-nnett).

Before

$ ipadecrypt bootstrap

▎ Step 1/4 · Sign in to the App Store
  ipadecrypt requires an Apple ID to download .ipas.
  It has to the be same Apple ID used on the jailbroken device.
  Credentials are stored locally on this machine.
  ▸ Apple ID email: email@example.com
  ▸ Apple ID password: ********
  ✗ login failed: login failed

After

$ ipadecrypt bootstrap         

▎ Step 1/4 · Sign in to the App Store
  ipadecrypt requires an Apple ID to download .ipas.
  It has to the be same Apple ID used on the jailbroken device.
  Credentials are stored locally on this machine.
  ▸ Apple ID email: email@example.com
  ▸ Apple ID password: ********
  ✓ authenticated
    Apple ID:    email@example.com
    Name:        Example Name
...

[londek]

LGTM! Thanks!