Alter ensure_sync implementation to support extensions

This allows extensions to override the Flask.ensure_sync method and
have the change apply to blueprints as well. Without this change it is
possible for differing blueprints to have differing ensure_sync
approaches depending on the extension used - which would likely result
in event-loop blocking issues.

This also allows blueprints to have a custom ensure_sync, although
this is a by product rather than an expected use case.

src/flask/helpers.py

@@ -755,6 +755,7 @@ def run_async(func):
         ctx module, except it has an async inner.
         """
         ctx = None
+
         if _request_ctx_stack.top is not None:
             ctx = _request_ctx_stack.top.copy()