client: pxar: add helper to handle optional preludes

client: pxar: add helper to handle optional preludes

Pxar archives with format version 2 allows to store optional
information file format version and prelude entries.

Cover the case for these entries, the file format version entry being
introduced to distinguish between different file formats used for
encoding as well as the prelude entry used to store optional metadata
such as the pxar cli exlude parameters.

Add the logic to accept and decode these prelude entries when
accessing the archive via a decoder instance.

For now simply ignore them.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>