Support external file system targets (CDN)

[aertmann]

Add support for external file systems targets (CDN) like S3, CloudFlare, Cloud Storage

Can be implemented by checking if the target is an instance of FileSystemTarget. Otherwise it needs to create a local copy of the resource, optimize it and publish/replace it in the target. Possible making it blocking.

[aertmann]

I have no plans of implementing this feature, but will happily accept pull requests for it. Would also be happy to do implement it upon request if anyone wants to sponsor it. A few hours of work would be needed.