feat: Implement PEPPOL and XRechnung validators for compliance with e-invoice specifications

- Added PeppolValidator class to validate PEPPOL BIS 3.0 invoices, including checks for endpoint IDs, document type IDs, process IDs, party identification, and business rules.
- Implemented validation for GLN check digits, document types, and transport protocols specific to PEPPOL.
- Added XRechnungValidator class to validate XRechnung 3.0 invoices, focusing on German-specific requirements such as Leitweg-ID, payment details, seller contact, and tax registration.
- Included validation for IBAN and BIC formats, ensuring compliance with SEPA regulations.
- Established methods for checking B2G invoice indicators and validating mandatory fields for both validators.

ts/interfaces/en16931-metadata.ts

@@ -23,6 +23,13 @@ export interface IEInvoiceMetadata {
   paidAmount?: number;                          // BT-113
   amountDue?: number;                           // BT-115
   
+  // Tax identifiers
+  sellerTaxId?: string;                         // BT-31
+  buyerTaxId?: string;                          // BT-48
+  buyerReference?: string;                      // BT-10
+  profileId?: string;                           // BT-23
+  paymentTerms?: string;                        // BT-20
+  
   // Delivery information (BG-13)
   deliveryAddress?: {
     streetName?: string;