Update type check in `get_user_function` to accept Callable instead of only types.FunctionType

[junah201]

I am making an adapter for running ASGI applications in GCP Cloud Function. (Repo)
Simply, I want to run my ASGI application (such as FastAPI or Django) in Cloud Function.

My adapter(Vellox) works like this:

from fastapi import FastAPI
from vellox import Vellox

app = FastAPI()


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: str = None):
    return {"item_id": item_id, "q": q}

vellox = Vellox(app=app, lifespan="off")

def handler(request):
    return vellox(request)

I confirmed that my adapter (vellox) works "hello world" example properly. But functions_framework throw error when handler is not types.FunctionType. In abvoe code, handler is instance of Vellox and also it is callable.
The code below is the part that is problematic.

# src/functions_framework/_function_registry.py

def get_user_function(source, source_module, target):
    """Returns user function, raises exception for invalid function."""

    # . . .

    function = getattr(source_module, target)
    # Check that it is a function
    if not isinstance(function, types.FunctionType):
        raise InvalidTargetTypeException(
            "The function defined in file {source} as '{target}' needs to be of "
            "type function. Got: invalid type {target_type}".format(
                source=source, target=target, target_type=type(function)
            )
        )
    return function

Instead of forcing the function type to be function, I suggest changing it to Callable, which has a slightly wider scope.