Support async template context processors

This is useful as there is contextual information that could be loaded
via IO e.g. information from a database. This also matches Quart and
hence makes the shared signature easier to manage.

src/flask/app.py

@@ -478,7 +478,7 @@ class Flask(App):
         for name in names:
             if name in self.template_context_processors:
                 for func in self.template_context_processors[name]:
-                    context.update(func())
+                    context.update(self.ensure_sync(func)())
 
         context.update(orig_ctx)