Allow vendor-specific info items to be stored in the Voucher

[EskoDijk]

As discussed in some of the Tuesday calls: since the voucher is created by the manufacturer (server), for consumption by the Pledge (from the same manufacturer), it's easy to include vendor-specific flags, fields and configuration elements in the Voucher.

It's a secure (signed) way to transport such configuration back to the Pledge already before it joins a prospective network.

How to encode this in the Voucher YANG definition? (Is it even possible with YANG?)