Update friends-cors to use writeFiles steps

[bph]

current two steps

{
			"step": "mkdir",
			"path": "wordpress/wp-content/mu-plugins"
		},
		{
			"step": "writeFile",
			"path": "wordpress/wp-content/mu-plugins/addFilter-0.php",
			"data": "<?php add_action( 'requests-requests.before_request', function( &$url ) {\n$url = 'https://playground.wordpress.net/cors-proxy.php?' . $url;\n} );"
		},

Become one step

{
  "step": "installPlugin",
  "pluginData": {
    "resource": "literal:directory",
    "name": "addFilter",
    "files": {
      "addFilter-0.php": "<?php add_action( 'requests-requests.before_request', function( &$url ) {\n$url = 'https://playground.wordpress.net/cors-proxy.php?' . $url;\n} );"
    }
  }
}

Also, while working on it let's use the plugins shorthand for the plugins installation of the activityPub and 'friends':

"plugins": ["activitypub", "friends"]