Unique User Id across Network

[dubeyAyush]

https://github.com/Beckn-One/DEG/blob/405fd47f7f209f84f94169e58a3cc8832ee0431a/docs/implementation-guides/v2/EV_Charging_V0.8-draft.md?plain=1#L999

The beckn:id seems to be for a given user but it's not defined whether it's unique for the whole network or just the BAP emitting out their userId

Also, the beckn:buyer schema is not defined