✨ Xero Integration (#639)

* ✨ Xero Integration

* ⚡ Add contact resource

* 🐛 Small fix

packages/nodes-base/credentials/XeroOAuth2Api.credentials.ts

@@ -0,0 +1,51 @@
+import {
+	ICredentialType,
+	NodePropertyTypes,
+} from 'n8n-workflow';
+
+const scopes = [
+	'offline_access',
+	'accounting.transactions',
+	'accounting.settings',
+	'accounting.contacts',
+];
+
+export class XeroOAuth2Api implements ICredentialType {
+	name = 'xeroOAuth2Api';
+	extends = [
+		'oAuth2Api',
+	];
+	displayName = 'Xero OAuth2 API';
+	properties = [
+		{
+			displayName: 'Authorization URL',
+			name: 'authUrl',
+			type: 'hidden' as NodePropertyTypes,
+			default: 'https://login.xero.com/identity/connect/authorize',
+		},
+		{
+			displayName: 'Access Token URL',
+			name: 'accessTokenUrl',
+			type: 'hidden' as NodePropertyTypes,
+			default: 'https://identity.xero.com/connect/token',
+		},
+		{
+			displayName: 'Scope',
+			name: 'scope',
+			type: 'hidden' as NodePropertyTypes,
+			default: scopes.join(' '),
+		},
+		{
+			displayName: 'Auth URI Query Parameters',
+			name: 'authQueryParameters',
+			type: 'hidden' as NodePropertyTypes,
+			default: '',
+		},
+		{
+			displayName: 'Authentication',
+			name: 'authentication',
+			type: 'hidden' as NodePropertyTypes,
+			default: 'header',
+		},
+	];
+}