Implement Declarative Function Signatures

[grant]

This issue is tracking implementing declarative function signatures in python.

This feature allows a functions framework to optionally detect the user code's function signature without specifying a --signature-type / FUNCTION_SIGNATURE_TYPE config. It may require a code change for the user.

A possible implementation for python may look like this:

@functions_framework.http("hello")
def hello(request):
    return "Hello world!"

@functions_framework.cloudevent("ce")
def function_name_does_not_matter(cloudevent):
    print(f"Received event with ID: {cloudevent['id']}")

The function (i.e. hello) would be invoked at localhost:8080/hello.

The functions-framework CLI would load all/any registered functions if available.

Note: The function name would not change the URL route or be used.

I think a decorator like this would be most clear, but other designs would be fine. As noted in the FF contract, event is not a requirement.

See:

• Node implementation: feat!: introduce declarative function signatures functions-framework-nodejs#347
• Contract update: https://github.com/GoogleCloudPlatform/functions-framework#specification-summary

CC: @matthewrobertson @anniefu

[grant]

Also, feel free to refactor

https://github.com/GoogleCloudPlatform/functions-framework-python/blob/master/src/functions_framework/__init__.py

if it's getting a bit too large of a surface area.