Simplify the async handling code

Firstly `run_sync` was a misleading name as it didn't run anything,
instead I think `async_to_sync` is much clearer as it converts a
coroutine function to a function. (Name stolen from asgiref).

Secondly trying to run the ensure_sync during registration made the
code more complex and brittle, e.g. the _flask_async_wrapper
usage. This was done to pay any setup costs during registration rather
than runtime, however this only saved a iscoroutne check. It allows
the weirdness of the Blueprint and Scaffold ensure_sync methods to be
removed.

Switching to runtime ensure_sync usage provides a method for
extensions to also support async, as now documented.

src/flask/helpers.py

@@ -6,7 +6,6 @@ import typing as t
 import warnings
 from datetime import timedelta
 from functools import update_wrapper
-from functools import wraps
 from threading import RLock
 
 import werkzeug.utils
@@ -803,10 +802,15 @@ def is_ip(value: str) -> bool:
     return False
 
 
-def run_async(func: t.Callable[..., t.Coroutine]) -> t.Callable[..., t.Any]:
-    """Return a sync function that will run the coroutine function *func*."""
+def async_to_sync(func: t.Callable[..., t.Coroutine]) -> t.Callable[..., t.Any]:
+    """Return a sync function that will run the coroutine function *func*.
+
+    This can be used as so
+
+        result = async_to_async(func)(*args, **kwargs)
+    """
     try:
-        from asgiref.sync import async_to_sync
+        from asgiref.sync import async_to_sync as asgiref_async_to_sync
     except ImportError:
         raise RuntimeError(
             "Install Flask with the 'async' extra in order to use async views."
@@ -818,9 +822,4 @@ def run_async(func: t.Callable[..., t.Coroutine]) -> t.Callable[..., t.Any]:
             "Async cannot be used with this combination of Python & Greenlet versions."
         )
 
-    @wraps(func)
-    def wrapper(*args: t.Any, **kwargs: t.Any) -> t.Any:
-        return async_to_sync(func)(*args, **kwargs)
-
-    wrapper._flask_sync_wrapper = True  # type: ignore
-    return wrapper
+    return asgiref_async_to_sync(func)