Related to #1787, Follows up on #1793

Implements GitDirectoryResource to enable loading files directly from
git repositories as follows:

```ts
{
	"landingPage": "/guides/for-plugin-developers.md",
	"steps": [
		{
			"step": "writeFiles",
			"writeToPath": "/wordpress/guides",
			"filesTree": {
				"resource": "git:directory",
				"url": "https://github.com/WordPress/wordpress-playground.git",
				"ref": "trunk",
				"path": "packages/docs/site/docs/main/guides"
			}
		}
	]
}
```

 ## Implementation details

Uses git client functions merged in
#1764 to sparse
checkout the requested files. It also leans on the PHP CORS proxy which
is now started as a part of the `npm run dev` command.

The CORS proxy URL is configurable per `compileBlueprint()` call so that each
Playground runtime may choose to either use it or not. For example, it
wouldn't be very useful in the CLI version of Playground.

 ## Testing plan

Go to
`http://localhost:5400/website-server/#{%20%22landingPage%22:%20%22/guides/for-plugin-developers.md%22,%20%22steps%22:%20[%20{%20%22step%22:%20%22writeFiles%22,%20%22writeToPath%22:%20%22/wordpress/guides%22,%20%22filesTree%22:%20{%20%22resource%22:%20%22git:directory%22,%20%22url%22:%20%22https://github.com/WordPress/wordpress-playground.git%22,%20%22ref%22:%20%22trunk%22,%20%22path%22:%20%22packages/docs/site/docs/main/guides%22%20}%20}%20]%20}`
and confirm Playground loads a markdown file.