[Blueprints] Add a github:artifact resource type

[adamziel]

GitHub artifacts are difficult to use as Blueprint resources. As doubly zipped archives, they need convoluted peeling. Let's introduce a new resource type called github:artifact that would take handle the peeling for the developer.

What I want the Blueprint to look like

{
	"steps": [
		{
			step: 'installPlugin',
			pluginData: {
				resource: 'github:artifact',
				url: zipArtifactUrl
			},
		}
	]
}

What the Blueprint looks like today

{
	"steps": [
		{
			step: 'mkdir',
			path: '/wordpress/pr',
		},
		/*
		 * This is the most important step.
		 * It download the built plugin zip file exposed by GitHub CI.
		 *
		 * Because the zip file is not publicly accessible, we use the
		 * plugin-proxy API endpoint to download it. The source code of
		 * that endpoint is available at:
		 * https://github.com/WordPress/wordpress-playground/blob/trunk/packages/playground/website/public/plugin-proxy.php
		 */
		{
			step: 'writeFile',
			path: '/wordpress/pr/pr.zip',
			data: {
				resource: 'url',
				url: zipArtifactUrl,
				caption: `Downloading Gutenberg PR ${prNumber}`,
			},
			progress: {
				weight: 2,
				caption: `Applying Gutenberg PR ${prNumber}`,
			},
		},
		/**
		 * GitHub CI artifacts are doubly zipped:
		 *
		 * pr.zip
		 *    gutenberg.zip
		 *       gutenberg.php
		 *       ... other files ...
		 *
		 * This step extracts the inner zip file so that we get
		 * access directly to gutenberg.zip and can use it to
		 * install the plugin.
		 */
		{
			step: 'unzip',
			zipPath: '/wordpress/pr/pr.zip',
			extractToPath: '/wordpress/pr',
		},
		{
			step: 'installPlugin',
			pluginData: {
				resource: 'vfs',
				path: '/wordpress/pr/gutenberg.zip',
			},
		}
	]
}

[adamziel]

Explorations in #1799