format/encoder/decoder: new pxar entry type `Version`

format/encoder/decoder: new pxar entry type `Version`

Introduces a new pxar format entry type `Version` and the associated
encoder and decoder methods. The format version entry is only allowed
once, as the first entry of the pxar archive, marked with a
`PXAR_FORMAT_VERSION` header followed by the encoded version number.
If not present, the default format version 1 is assumed as encoding
format for the archive.

The entry allows to early detect incompatibility with an encoded
archive and bail or switch mode based on the encountered version.

The format version entry is not backwards compatible to pxar format
version 1.

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