Add support for Proposed type v8 (fully custom)

[cowtowncoder]

So as per:

• https://uuid6.github.io/uuid6-ietf-draft/
• https://www.ietf.org/archive/id/draft-peabody-dispatch-new-uuid-format-04.html

there are 3 types; this is for implementing variant 8, DIY variant where actual definition of structure is not specified by spec; this can be used for fully custom uuids where semantics are specific to use case / context.

It may be that this just means ability to accept such uuids, or something more, depending on what makes most sense.