Create test module to enable TUF validation in Package Manager

[tedbow]

Problem

Re: [policy, no patch] How much of The Update Framework integration is needed for alpha-level review/commit of Package Manager?

We are going to use this project to have core reviewers be able to test TUF integration in Package Manager/Automatic Updates while still making a requirement the Alpha version of the core experimental module.

While just using this project will make the repo TUF protected it will not flag a validation error in Package Manager if you disable that setting

Propose solution

Create a test Drupal module that will enable \Drupal\package_manager\Validator\PhpTufValidator and changes repo it protects to https://drupal:drupal@packages.staging.devdrupal.org/8

We will need some changes to PhpTufValidator so that we can change the repo URL. This will need to be done in the Package Manager module itself(issue not yet created)