In order to accommodate biscuit issuers with multiple key pairs in
use, whether concurrently or in an ongoing rotation cycle, biscuits
can record and expose an identifier for the root private key used to
sign its authority block. Allow issuers to associate such an
identifier with the private key when creating a new biscuit.

Introduce the option function "WithRootKeyID" to supply such an
identifier at composition time, and the "(*Biscuit).RootKeyID" method
to query this identifier later.