[5.7] Expose readStream and writeStream to allow cross-filesystem actions

[tomlankhorst]

This allows easy reading/writing of files across filesystems and to response-streams.

Across filesystems:

Storage::disk('s3')
  ->writeStream(
    'remote-file.zip',
    Storage::disk('local')->readStream('local-file.zip')
  );

To response-streams:

return response()
  ->stream( function() {
    fpassthru( Storage::disk('s3')->readStream('file.zip') );
  });

[sisve]

Would it make sense to have readStream throw an exception on failures instead of returning null? A null value cannot provide much debugging information into what failed.

[taylorotwell]

Why are we inventing new exception types. Just let Flysystem throw whatever exception it throws.

[tomlankhorst]

@taylorotwell Such that I can specify what exceptions the implementation of the Filesystem contract might throw - analogous to FileNotFoundException - the handle them appropriately.

[tomlankhorst]

@sisve Sure, currently this is a relic of Flysystem which returns null on failure. Could throw an exception on null return as well.