[Core] Check is async callable (#21714)

To permit proper coercion of objects like the following:


```python
class MyAsyncCallable:
    async def __call__(self, foo):
        return await ...

class MyAsyncGenerator:
    async def __call__(self, foo):
        await ...
        yield 
```

libs/core/tests/unit_tests/runnables/test_runnable.py

@@ -4883,6 +4883,23 @@ async def test_runnable_gen() -> None:
     assert [p async for p in arunnable.astream(None)] == [1, 2, 3]
     assert await arunnable.abatch([None, None]) == [6, 6]
 
+    class AsyncGen:
+        async def __call__(self, input: AsyncIterator[Any]) -> AsyncIterator[int]:
+            yield 1
+            yield 2
+            yield 3
+
+    arunnablecallable = RunnableGenerator(AsyncGen())
+    assert await arunnablecallable.ainvoke(None) == 6
+    assert [p async for p in arunnablecallable.astream(None)] == [1, 2, 3]
+    assert await arunnablecallable.abatch([None, None]) == [6, 6]
+    with pytest.raises(NotImplementedError):
+        arunnablecallable.invoke(None)
+    with pytest.raises(NotImplementedError):
+        arunnablecallable.stream(None)
+    with pytest.raises(NotImplementedError):
+        arunnablecallable.batch([None, None])
+
 
 async def test_runnable_gen_context_config() -> None:
     """Test that a generator can call other runnables with config