[Proposal] Add download and response methods to Filesystem interface

[wgriffioen]

The interface \Illuminate\Contracts\Filesystem\Filesystem does not contain all the methods which exist in the implementation in \Illuminate\Filesystem\FilesystemAdapter.

If you're using contracts instead of the facades, which I do a lot, PHPStorm gives me a warning about a method not existing on the interface as can be seen in the following screenshot:

In the contract reference it says that Storage::disk() is a reference to \Illuminate\Contracts\Filesystem\Filesystem, but with the missing methods, it's not a complete reference.

I'm willing to add those methods to the interface, but before I do this I'd like to know if there are any reasons why this shouldn't be done or why this hasn't been done already. I've seen methods have been added in the past, so I figured I'd open this proposal.