[12.x] Add attachment helper method to add attachment from cloud storage

[PhiloNL]

Laravel already has a convenient helper like Storage::cloud() to have a simple and unified approach when dealing with the apps main cloud based file system. This PR adds a new addition to make it easier to add attachments to emails that are stored on the cloud.

// Before:
Attachment::fromStorageDisk(Storage::getDefaultCloudDriver(), $attachment->path);

// After:
Attachment::fromCloudStorage($attachment->path);