Mint a scoped API key

The scoped grant for the new key: a coarse scopes set (what the key may do), an optional numberScope phone-number allow-list (omit or leave empty to allow every number in the workspace), and an optional per-key spendLimitCents cap with an optional spendLimitResetPeriod (monthly or lifetime). Each of these must stay within the minting key's own ceiling. The plaintext key is returned exactly once, in the response to this call.