✨ Xero Integration (#639)

* ✨ Xero Integration

* ⚡ Add contact resource

* 🐛 Small fix

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

@@ -0,0 +1,76 @@
+import {
+	OptionsWithUri,
+ } from 'request';
+
+import {
+	IExecuteFunctions,
+	IExecuteSingleFunctions,
+	ILoadOptionsFunctions,
+} from 'n8n-core';
+
+import {
+	IDataObject,
+} from 'n8n-workflow';
+
+export async function xeroApiRequest(this: IExecuteFunctions | IExecuteSingleFunctions | ILoadOptionsFunctions, method: string, resource: string, body: any = {}, qs: IDataObject = {}, uri?: string, headers: IDataObject = {}): Promise<any> { // tslint:disable-line:no-any
+	const options: OptionsWithUri = {
+		headers: {
+			'Content-Type': 'application/json',
+		},
+		method,
+		body,
+		qs,
+		uri: uri || `https://api.xero.com/api.xro/2.0${resource}`,
+		json: true
+	};
+	try {
+		if (body.organizationId) {
+			options.headers = { ...options.headers, 'Xero-tenant-id': body.organizationId };
+			delete body.organizationId;
+		}
+		if (Object.keys(headers).length !== 0) {
+			options.headers = Object.assign({}, options.headers, headers);
+		}
+		if (Object.keys(body).length === 0) {
+			delete options.body;
+		}
+		//@ts-ignore
+		return await this.helpers.requestOAuth2.call(this, 'xeroOAuth2Api', options);
+	} catch (error) {
+		let errorMessage;
+
+		if (error.response && error.response.body && error.response.body.Message) {
+
+			errorMessage = error.response.body.Message;
+
+			if (error.response.body.Elements) {
+				const elementErrors = [];
+				for (const element of error.response.body.Elements) {
+					elementErrors.push(element.ValidationErrors.map((error: IDataObject) =>  error.Message).join('|'));
+				}
+				errorMessage = elementErrors.join('-');
+			}
+			// Try to return the error prettier
+			throw new Error(`Xero error response [${error.statusCode}]: ${errorMessage}`);
+		}
+		throw error;
+	}
+}
+
+export async function xeroApiRequestAllItems(this: IExecuteFunctions | ILoadOptionsFunctions, propertyName: string ,method: string, endpoint: string, body: any = {}, query: IDataObject = {}): Promise<any> { // tslint:disable-line:no-any
+
+	const returnData: IDataObject[] = [];
+
+	let responseData;
+	query.page = 1;
+
+	do {
+		responseData = await xeroApiRequest.call(this, method, endpoint, body, query);
+		query.page++;
+		returnData.push.apply(returnData, responseData[propertyName]);
+	} while (
+		responseData[propertyName].length !== 0
+	);
+
+	return returnData;
+}