Provide an extendable merge blueprint funcs method

This allows a Blueprint implementation that has additional funcs, such
as Quart with its before_websocket_funcs (as example), to extend the
merge method to also merge these functions.

src/flask/sansio/blueprints.py

@@ -329,37 +329,7 @@ class Blueprint(Scaffold):
 
         # Merge blueprint data into parent.
         if first_bp_registration or first_name_registration:
-
+            self._merge_blueprint_funcs(app, name)
-            def extend(bp_dict, parent_dict):
-                for key, values in bp_dict.items():
-                    key = name if key is None else f"{name}.{key}"
-                    parent_dict[key].extend(values)
-
-            for key, value in self.error_handler_spec.items():
-                key = name if key is None else f"{name}.{key}"
-                value = defaultdict(
-                    dict,
-                    {
-                        code: {
-                            exc_class: func for exc_class, func in code_values.items()
-                        }
-                        for code, code_values in value.items()
-                    },
-                )
-                app.error_handler_spec[key] = value
-
-            for endpoint, func in self.view_functions.items():
-                app.view_functions[endpoint] = func
-
-            extend(self.before_request_funcs, app.before_request_funcs)
-            extend(self.after_request_funcs, app.after_request_funcs)
-            extend(
-                self.teardown_request_funcs,
-                app.teardown_request_funcs,
-            )
-            extend(self.url_default_functions, app.url_default_functions)
-            extend(self.url_value_preprocessors, app.url_value_preprocessors)
-            extend(self.template_context_processors, app.template_context_processors)
 
         for deferred in self.deferred_functions:
             deferred(state)
@@ -406,6 +376,36 @@ class Blueprint(Scaffold):
             bp_options["name_prefix"] = name
             blueprint.register(app, bp_options)
 
+    def _merge_blueprint_funcs(self, app: App, name: str) -> None:
+        def extend(bp_dict, parent_dict):
+            for key, values in bp_dict.items():
+                key = name if key is None else f"{name}.{key}"
+                parent_dict[key].extend(values)
+
+        for key, value in self.error_handler_spec.items():
+            key = name if key is None else f"{name}.{key}"
+            value = defaultdict(
+                dict,
+                {
+                    code: {exc_class: func for exc_class, func in code_values.items()}
+                    for code, code_values in value.items()
+                },
+            )
+            app.error_handler_spec[key] = value
+
+        for endpoint, func in self.view_functions.items():
+            app.view_functions[endpoint] = func
+
+        extend(self.before_request_funcs, app.before_request_funcs)
+        extend(self.after_request_funcs, app.after_request_funcs)
+        extend(
+            self.teardown_request_funcs,
+            app.teardown_request_funcs,
+        )
+        extend(self.url_default_functions, app.url_default_functions)
+        extend(self.url_value_preprocessors, app.url_value_preprocessors)
+        extend(self.template_context_processors, app.template_context_processors)
+
     @setupmethod
     def add_url_rule(
         self,