feat(run): add helloworld

[grayside]

This PR adds the first primary Cloud Run sample to the php-docs-samples repo.

• This is a helloworld sample and will replace the knative-hosted sample at https://cloud.google.com/run/docs/quickstarts/build-and-deploy#php
• Includes an end-to-end test based on php-tools. It uses ID Token minting support from google/auth, and adds google/auth as a global testing dependency.
• Proposes a baseline Dockerfile which will be visible in the quickstart: it balances simplicity with Cloud Run runtime optimization by adding opcache and customizing some PHP settings. More advanced topics such as database support or composer builds are left for more specialized samples.
• Adds a Cloud Run "landing" README along the lines of what Node.js uses.

[averikitsch]

Does this need to include <?php?

[grayside]

PHP tag must contain the comment.