Simplify fiat amount usage

[bumi]

Using fiat amounts is still a bit complicated.

maybe we can simpify this:

const amount = await fiat.getSatoshiValue({
  amount: 0.1,
  currency: "usd",
});

const invoice = await nwcClient.makeInvoice({
  amount: amount * 1000,
  description: "Video access",
});

const invoice = await nwcClient.makeInvoice({
  amount: USD(0.1),
  description: "Video access",
});

USD is an object that can return sats and millisats values and the SDK checks if it gets a number or an object that has a toMilliSats() function or something.

this USD implementation might need to go into lightning-tools ?