Add `source` parameter to `directory`

[puetzp]

This is similar to the source parameter in the file resource, which contains a path to an asset on the server. When used in a file resource, pullconf downloads the file from pullconfd at the given path.

The source parameter should be added to the directory resource to enable users to copy whole directories to a client.

This should not collide with the existing logic regarding the purge parameter:

• if purge is set to true and the directory is downloaded from the server, the client removes every file that is not part of the remote directory.
• if purge is set to false and the directory is downloaded from the server, the client only checks if the files that reside on the remote server still match the local version, and ignores the other files.

Possible issues: How to transfer ownership and file mode metadata for the files that reside in the remote directory being copied.