Support a playground.json file in git repositories

[adamziel]

Once we can install WordPress plugins and themes directly from Github, the developers will want to preconfigure how Playground handles their repositories, e.g. which Github directories are mounted in which Playground directories.

A playground.json file support in the GitHub repository could provide such a way.

• @ndiego and @justintadlock have insights about a playground.json file, in particular I remember it needed a special naming scheme or else certain GitHub URLs wouldn't work (or raw.githubusercontent.com?)
• @tellyworth has insights from setting up Blueprints support in the WordPress.org plugins directory. In particular, we may need to support a playground subdirectory to enable bundling additional Playground-specific assets.
• The Blueprints gallery should use the same structure as we invent here. It would be just a special case of a multi-blueprint repository.

This could be a good opportunity to explore how such a file could converge with Playground runtime configuration file and, potentially, wp-env.json.

[adamziel]

Related to #1731 that introduces v1 of the runtime configuration format. Also related to WordPress/blueprints-library#1 and WordPress/blueprints-library#6