Method to get EndpointID for supplier

This can be used as an unique identifier for each supplier,
e.g. as EndUserID when reporting end user statistics to
OpenPEPPOL

api/src/main/java/peppol/bis/invoice3/api/PeppolBillingApi.java

@@ -16,6 +16,7 @@
 package peppol.bis.invoice3.api;
 
 import org.eaxy.Document;
+import org.eaxy.Element;
 import peppol.bis.invoice3.domain.BillingCommon;
 import peppol.bis.invoice3.domain.CreditNote;
 import peppol.bis.invoice3.domain.Invoice;
@@ -85,6 +86,14 @@ public String getCustomerCountryIdentifier() {
         throw new RuntimeException("Mandatory property missing in document: AccountingCustomerParty -> Party -> PostalAddress -> Country -> IdentificationCode");
     }
 
+    public String getSupplierEndpointID() {
+        if (this.object instanceof Document) {
+            Element element = ((Document) this.object).find("AccountingSupplierParty", "Party", "EndpointID").single();
+            return element.attr("schemeID").trim() + ":" + element.text().trim();
+        }
+        throw new RuntimeException("Mandatory property missing in document: AccountingSupplierParty -> Party -> EndpointID");
+    }
+
     public String prettyPrint() {
         return XML_FIRST_LINE + (this.object instanceof BillingCommon ? ((BillingCommon) this.object).xmlRoot() : ((Document) this.object).getRootElement()).toIndentedXML();
     }

api/src/test/java/peppol/bis/invoice3/api/PeppolBillingApiTest.java

@@ -35,6 +35,7 @@ public void test() throws IOException {
             PeppolBillingApi<Document> peppolBillingApi = new PeppolBillingApi<>(document);
             assertEquals("NO", peppolBillingApi.getCustomerCountryIdentifier());
             assertEquals("NO", peppolBillingApi.getSupplierCountryIdentifier());
+            assertEquals("0192:123456785", peppolBillingApi.getSupplierEndpointID());
         }
     }