Add support for WebDAV (RFC 4918)

[Kaliumhexacyanoferrat]

As a developer of a web application, I would like to provide content via WebDAV so that client applications can consume this content in a file based way.

Example

var provider = WebDAV.From("./dav").ReadOnly();

Acceptance criteria

• The provider decouples the protocol from the actual data source (e.g. files and folders)
• There is an easy to use implementation for file system based access
• Features required by the implementation (e.g. handling of ETags) are not coupled with the WebDAV provider and are solved in separate issues
• The implementation supports at least class 1 of the RFC