Are part of WiredTiger design for tiered storage, we'll need a way to encapsulate uses of external storage (e.g. AWS, Google Cloud, Azure). This ticket will be to flesh out what an "external storage extension" would look like (perhaps akin to WT_FILE_SYSTEM or WT_ENCRYPTOR?), and how it would operate. One part of this will be to show how authentication with a cloud provider could be handled.