Feat: Add Fiat enum

[IllegalWeaponsDealer]

Hello,

I noticed there’s no Fiat enum, although Currency is present.
I’ve taken the list of enums from Tatum’s supported fiats and turned them into a file.

In my opinion, this makes things more type-safe instead of relying on plain strings.

Is it possible to get this merged?

OLD:

const rates = await this.btcSdk.rates.getCurrentRateBatch([
    { basePair: "EUR", currency: Currency.BTC },
    { basePair: "EUR", currency: Currency.LTC },
    { basePair: "EUR", currency: Currency.ETH },
    { basePair: "EUR", currency: Currency.SOL }
]);

NEW:

const rates = await this.btcSdk.rates.getCurrentRateBatch([
    { basePair: Fiat.EUR, currency: Currency.BTC },
    { basePair: Fiat.EUR, currency: Currency.LTC },
    { basePair: Fiat.EUR, currency: Currency.ETH },
    { basePair: Fiat.EUR, currency: Currency.SOL }
]);