webhooks: Migrate helloworld to use WildValue to use @typed_endpoint.

We owe more documentation on the use of WildValue. A follow-up on
updating it with examples of WildValue and endpoint will be desirable.

zerver/webhooks/helloworld/view.py

@@ -1,21 +1,22 @@
 # Webhooks for external integrations.
-from typing import Any, Dict, Sequence
 
 from django.http import HttpRequest, HttpResponse
 
 from zerver.decorator import webhook_view
-from zerver.lib.request import REQ, has_request_variables
 from zerver.lib.response import json_success
+from zerver.lib.typed_endpoint import WebhookPayload, typed_endpoint
+from zerver.lib.validator import WildValue, check_string
 from zerver.lib.webhooks.common import check_send_webhook_message
 from zerver.models import UserProfile
 
 
 @webhook_view("HelloWorld")
-@has_request_variables
+@typed_endpoint
 def api_helloworld_webhook(
     request: HttpRequest,
     user_profile: UserProfile,
-    payload: Dict[str, Sequence[Dict[str, Any]]] = REQ(argument_type="body"),
+    *,
+    payload: WebhookPayload[WildValue],
 ) -> HttpResponse:
     # construct the body of the message
     body = "Hello! I am happy to be here! :smile:"
@@ -24,7 +25,10 @@ def api_helloworld_webhook(
     body_template = (
         "\nThe Wikipedia featured article for today is **[{featured_title}]({featured_url})**"
     )
-    body += body_template.format(**payload)
+    body += body_template.format(
+        featured_title=payload["featured_title"].tame(check_string),
+        featured_url=payload["featured_url"].tame(check_string),
+    )
 
     topic = "Hello World"