Support callable http functions

Would be useful to have callable function out of the box. [Docs](https://firebase.google.com/docs/functions/callable), [Specs](https://firebase.google.com/docs/functions/callable-reference)

Start by reading the linked Firebase callable-functions documentation and callable reference to identify the required behavior. Then inspect the framework's HTTP function handling and determine which entry points and tests would need to change; done means callable HTTP functions work according to the referenced specification.

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