Allow multibase encoding for driver's license VCBs.

[wes-smith]

The spec currently recommends base64url for driver's license VCBs - the text should explicitly allow the use of multibase for better portability as well.

[dlongley]

Something like "A multibase header MAY be prepended to the base-encoded VCB data. PDF417 field header specifications are expected to indicate whether a multibase header is present or absent."