Change the signature of auth scripts to accept `FEE_ARGS` in addition to `AUTH_ARGS`

I think a cleaner approach would be to change the signature to accept "fee args" as a second word - i.g.:

```
pub type FeeArgs = struct { faucet_id: AccountId, rate: ConversionRate }
pub type ConversionRate = struct { lo: felt, hi: felt }

#! Inputs: [AUTH_ARGS, FEE_ARGS, pad(8)]
#! Outputs: [pad(16)]
#!
#! Invocation: call
@auth_script
pub proc auth_tx(auth_args: word, fee_args: FeeArgs)
...
end
```

This is a bit more work though - so, probably best to do in a follow-up.

_Originally posted by @bobbinth in https://github.com/0xMiden/protocol/pull/3298#discussion_r3583044185_