lib: add type for input/output variant differentiation

lib: add type for input/output variant differentiation

Introduce an enum which stores 2 different possible variants of
inputs or outputs to be passed to encoder and decoder/accessor
instances, depending whether to read/write a fully self contained
pxar archive or whether to split off the payload stream into a
separate input/output.

Co-authored-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Christian Ebner <c.ebner@proxmox.com>