Join tokens are created by the create_join_token() SQL builtin and
returned as base64 encoded strings.

• To account for any future
  modifications to the join token format, the marshalled form of it
  should have a version number in it at a fixed offset (ideally 0)
  in addition to all the existing fields. This will ease in any future
  updates to the join token format.
• the base64 format can produce strings that start with a hyphen (-) and this confuses the command-line parser when pasted in argument position on the command line for cockroach connect join.

Stems from #60632.
Epic: CRDB-6663