✨ Add Facebook Trigger Node (#1106)

* ⚡ Facebook trigger from webhooks

Uses tokens just like the GraphQL API but this one requires an app token
instead of an user access token.

* ⚡ Improvements
:zap Improvements

* 🐛 Fix credential file name

Co-authored-by: Omar Ajoue <krynble@gmail.com>

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

@@ -0,0 +1,21 @@
+import {
+	ICredentialType,
+	NodePropertyTypes,
+} from 'n8n-workflow';
+
+export class FacebookGraphSubscriptionApi implements ICredentialType {
+	name = 'facebookGraphSubscriptionApi';
+	displayName = 'Facebook Graph API';
+	extends = [
+		'facebookGraphApi',
+	];
+	properties = [
+		{
+			displayName: 'APP Secret',
+			name: 'appSecret',
+			type: 'string' as NodePropertyTypes,
+			default: '',
+			description: '(Optional) When the app secret is set the node will verify this signature to validate the integrity and origin of the payload.',
+		},
+	];
+}