generate: allow setting a root key id

[divarvel]

biscuit-cli on  root-key-id [$?] is 📦 v0.3.0 via 🦀 v1.68.1
❯ echo 'toto(true)'|biscuit generate --private-key-file private.key --root-key-id 1 - | biscuit inspect -
Open biscuit
Authority block (root key identifier: 1):
== Datalog ==
toto(true);

== Revocation id ==
1d888c544b44172b8262328cb2b76fc6512143a0984bb0c25f289007cd7b4268990d99aedff3a2ca06c0025ed48574401d7eca8467ed354198bd452e020d7e0c

==========

🙈 Public key check skipped 🔑
🙈 Datalog check skipped 🛡️