New dependency: UUID

[ocket 8888]

For rewriting the /user/reset_password endpoint (PR #3932) I was importing
the github.com/google/uuid library for generating UUIDs used as temporary
login tokens. That's backward-compatible with Perl (for all that matters).
The repo is licensed under BSD-3 which I think is Apache-compatible, but I
think bringing that to the attention of the mailing list is standard
procedure.