Adding `defaultFiatCurrency` query param support

Transak now supports the defaultFiatCurrency query param, allowing partners to set the default value for the Fiat currency when the widget is loaded.

GIVEN: defaultFiatCurrency = EUR

WHEN: The query params URL is

THEN: The Transak Widget will be loaded with EUR as the default fiat option.

Please check our docs for a detailed explanation and usage of defaultFiatCurrency query param.