Support stacked decorators for async workflows (#192)

Currently, You can't declare an async function to be both a FastAPI
handler and a DBOS workflow:

```python
@app.get("/endpoint/{var1}/{var2}")
@DBOS.workflow()
async def test_endpoint(var1: str, var2: str) -> str:
    return f"{var1}, {var2}!"
```

The problem stems from the function returned by the `@DBOS.workflow`
decorator. Both the decorated function and the function returned by the
decorator return a coroutine, but the function returned by the decorator
is not defined with `async def`. This causes FastAPI to mis-categorize
the workflow function as sync when it is actually async. The function
retuned by the decorator has to appear as a coroutine to
`inspect.iscoroutinefunction`.

For Python 3.12 and later, any function can be marked as a coroutine
using
[`inspect.markcoroutinefunction`](https://docs.python.org/3/library/inspect.html#inspect.markcoroutinefunction).

For Python 3.11 and earlier, we have to wrap the coroutine returning
function in an async function like this:

```python
def _mark_coroutine(func: Callable[P, R]) -> Callable[P, R]:
    @wraps(func)
    async def async_wrapper(*args: Any, **kwargs: Any) -> R:
        return await func(*args, **kwargs)  # type: ignore

    return async_wrapper  # type: ignore
```

dbos/_core.py

@@ -488,6 +488,22 @@ def start_workflow(
     return WorkflowHandleFuture(new_wf_id, future, dbos)
 
 
+if sys.version_info < (3, 12):
+
+    def _mark_coroutine(func: Callable[P, R]) -> Callable[P, R]:
+        @wraps(func)
+        async def async_wrapper(*args: Any, **kwargs: Any) -> R:
+            return await func(*args, **kwargs)  # type: ignore
+
+        return async_wrapper  # type: ignore
+
+else:
+
+    def _mark_coroutine(func: Callable[P, R]) -> Callable[P, R]:
+        inspect.markcoroutinefunction(func)
+        return func
+
+
 def workflow_wrapper(
     dbosreg: "DBOSRegistry",
     func: Callable[P, R],
@@ -548,7 +564,7 @@ def init_wf() -> Callable[[Callable[[], R]], R]:
         )
         return outcome()  # type: ignore
 
-    return wrapper
+    return _mark_coroutine(wrapper) if inspect.iscoroutinefunction(func) else wrapper
 
 
 def decorate_workflow(
@@ -838,6 +854,10 @@ def wrapper(*args: Any, **kwargs: Any) -> Any:
                 assert tempwf
                 return tempwf(*args, **kwargs)
 
+        wrapper = (
+            _mark_coroutine(wrapper) if inspect.iscoroutinefunction(func) else wrapper  # type: ignore
+        )
+
         def temp_wf_sync(*args: Any, **kwargs: Any) -> Any:
             return wrapper(*args, **kwargs)