Token format / construction

[moonshiner]

(from Erik)

It seems like the actual token contents should have more flexibility.
I don't think we want a "MUST" on that particular construct. It may be worth
a MUST that there is at least 128 bits of secure entropy, and that the token is
either base64 or hex encoded. But there may be a need to use other
constructs in the future (eg, not SHA256). Giving the current example
as a MAY seems reasonable.

There may be reasons for other constructs that embed state within the token.
For example: "HMAC-SHA256(private_key, label+account+domain)" may be appropriate
in some cases, although has enough security considerations that I'm not
sure we want to include that.