tools: add zip module

tools: add zip module

This modules contains the 'ZipEncoder' struct, which wraps an async writer,
to create a ZIP archive on the fly

To create a ZIP file, have a target that implements AsyncWrite,
give it to ZipEncoder::new, add entries via 'add_entry' and
at the end, call 'finish'

for now, this does not implement compression (uses ZIPs STORE mode), and
does not support empty directories or hardlinks (or any other special
files)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>