feat: Declarative function signatures for python

[KaylaNguyen]

Allow users to systematically register their function signature type:

import functions_framework 

@function_framework.cloudevent
def hello_cloudevent(cloudevent):
    print("Hello")

@function_framework.http
def hello_http(request):
    print("Hello")

User signature type is searched in the following order:

• Decorator user used to register their function (new behavior)
• --signature-type flag (current behavior)
• environment variable FUNCTION_SIGNATURE_TYPE (current behavior)

If none of the above is set, signature type defaults to be "http".

[grant]

Great PR. I've requested a few changes.

[grant]

Reviewing further, I don't think we want to support @function_framework.event, following the Node implementation and CloudEvent future interface for events.

[matthewrobertson]

LGTM!