Allow `MintNote` to carry multiple mint requests

Currently, the `MintNote` standard only carries a single mint request. With the faucet [migrating to a network account](https://github.com/0xMiden/faucet/issues/261#issuecomment-5096467395), a possible optimization could be to allow carrying multiple mint requests in a single note.

As I explain in the issue, I think this could diminish the state bloat from creating a single mint note just for a single asset note, which could be useful in various usecases, not just a testnet faucet. We could use note storage and/or attachments for this.