allow to configure a custom guzzle client for cache proxy requests

the LiipCacheControlBundle had an option for any custom curl headers. this seems the simplest solution to get the possibilities back without overcomplicating things. people will have to set up their own guzzle service though.
this means I would remove the tag from here. People using the misd bundle and wanting the behavior of the tag will know how to apply it.
Putting the tag here could be confusing for users