Added E2E encryption

The end goal for invoice.build is to store the invoice data completely onchain. For that to happen the data will have to be stored in some kind of encrypted hash.

Therefore, the first step towards moving the invoices onchain is allow the creation of invoices in the current API/database by simply providing a password and data hash. This is what I have just added.

Now, if you provided a password in the invoice settings you can choose to encrypt your invoice data E2E meaning that it's stored in our database as a random string. When you try to load this invoice on the frontend you will then need to provide the password again to unencrypt that data. This ensures invoice.build never sees or stores your invoice data.