Filesystem capabilities

[bentleyedwards]

Currently, Craft assumes that any filesystem can serve both as an asset filesystem and a transform filesystem. I'm developing a plugin for imgix that uses a custom filesystem, which can only function as a transform filesystem since imgix does not manage direct image uploads. It would be beneficial if filesystems could specify their capabilities—whether they can act as an asset filesystem, a transform filesystem, or both. This way, only filesystems with the appropriate capabilities would appear in the relevant dropdowns in the volumes setup screen. This could be implemented using a property like isSelectable, for instance.