Feature request: add function to call the Wiki Attachments - Get route

[FurcyPin]

The WikiClient has a create_attachment method that matches the Attachments - Create route of the API, but it seems that there is not get_attachment method to match the Attachments - Get route of the API.

Could you please add one ? While we're at it, it would be nice to have a kind of get-attachment-checksum API route to avoid re-uploading the same files when they don't change.