✨ Xero Integration (#639)

* ✨ Xero Integration

* ⚡ Add contact resource

* 🐛 Small fix

packages/nodes-base/nodes/Xero/InvoiceInterface.ts

@@ -0,0 +1,40 @@
+import {
+	IDataObject,
+ } from 'n8n-workflow';
+
+ export interface ILineItem {
+	Description?: string;
+	Quantity?: string;
+	UnitAmount?: string;
+	ItemCode?: string;
+	AccountCode?: string;
+	LineItemID?: string;
+	TaxType?: string;
+	TaxAmount?: string;
+	LineAmount?: string;
+	DiscountRate?: string;
+	Tracking?: IDataObject[];
+}
+
+export interface IInvoice extends ITenantId {
+	Type?: string;
+	LineItems?: ILineItem[];
+	Contact?: IDataObject;
+	Date?: string;
+	DueDate?: string;
+	LineAmountType?: string;
+	InvoiceNumber?: string;
+	Reference?: string;
+	BrandingThemeID?: string;
+	Url?: string;
+	CurrencyCode?: string;
+	CurrencyRate?: string;
+	Status?: string;
+	SentToContact?: boolean;
+	ExpectedPaymentDate?: string;
+	PlannedPaymentDate?: string;
+}
+
+export interface ITenantId {
+	organizationId?: string;
+}