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.
we are using misd/guzzle-bundle . the tag comes from https://github.com/misd-service-development/guzzle-bundle/blob/master/Resources/doc/clients.md but i don't know exactly what its there for or what it does.