initialize method
Initializes the manager with the merchant's identity.
sellerName - Merchant / taxpayer name as registered with ZATCA.
sellerTRN - 15-digit VAT registration number; must start and
end with 3.
Implementation
void initialize({required String sellerName, required String sellerTRN}) {
_validateSellerName(sellerName);
_validateSellerTRN(sellerTRN);
_sellerName = sellerName.trim();
_sellerTRN = sellerTRN;
}