x/net/webdav: FileSystem interface should include CopyFile

[fstanis]

WebDAV requires support for the COPY method (defined in RFC 2518 section 8.8). Go's WebDAV server implementation in golang.org/x/net/webdav implements it, however the current implementation simply calls FileSystem.OpenFile on the source and destination and uses io.Copy to pipe the content.

While this is fine in some cases, I really think that the user should be able to override / implement this by adding a CopyFile method to the FileSystem interface. Two major reasons I see:

• The current implementation is inefficient on filesystems that support copy-on-write.
• The current implementation is inefficient on remote filesystems (effectively, it will pull a file locally and send it back).

[toothrot]

I am not certain who owns x/net these days.
/cc @bradfitz @ianlancetaylor

[fstanis]

BTW happy to send a PR, just wanted to get an opinion on whether this kind of API change is a good idea first.