Given an account id, the user can add public keys. The added public keys can sign
intents on behalf of these accounts, even to add new ones.
Warning: Implicit account ids, by default, have their corresponding public keys added.
Meaning: For a leaked private key, whose implicit account id had been used in intents,
the user must manually rotate the underlying public key within intents, too.
Every intent execution requires a nonce. Each account id gets (over time, while using the intents contract) more nonces,
and this ensures that nonces are not reused to avoid replay attacks. This “marks” the nonce as used.