Before this change x/net/webdav would open each file to find out its
Content-Type.
Now we override the FileInfo and provide that directly from rclone.
An --etag-hash has also been implemented to override the ETag with the
hash passed in.
Fixes#2273
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.