api: query_url_metadata: optionally detect compression

api: query_url_metadata: optionally detect compression

extend the query_url_metadata endpoint with the option to detect and return
used compression algorithms, if supported by PVE. this will be used to support
decompression as part of the download flow for certain file types (ISO files
for now).

Signed-off-by: Philipp Hufnagl <p.hufnagl@proxmox.com>
Slightly reworded commit title/message
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by: Dominik Csapak <d.csapak@proxmox.com>