Add wildcard cache key deletion for device type cache purge

[pradeep910]

Adds wildcard check to delete device type cache such as <URL>--*.

This requires following filter as URLs will not contain --* by default.

add_filter( 'rt_nginx_helper_purge_url', function( $url ) {
       // Add wildcard in cache key URL for all device type keys deletion.
       $url = $url . '--*';
       return $url;
});

For #202

[radhe]

@kirtangajjar @th3karkota
can you do the testing?

[pradeep910]

We've merged this plugin after testing it on one of our live sites.
cc @chandrapatel @kirtangajjar @radhe