Improved cross-language homogeneity

[lkingland]

Summary

Use an explicit "handle" method on Function instances, as this is the most similar method signature approach between different language; a primary goal of the project. This is in contrast to a more strict adherence to the ASGI spec which would see the Function object itself be callable.

Rationale

Python's call is idiomatically used when the primary purpose of an object is to be callable with a single, well-defined operation. This matches ASGI's design philosophy where handlers are fundamentally single-purpose callables that process requests. However, the Function object also supports additional methods such as readiness/liveness checks, and start/stop event handlers. Furthermore, the cross-language design goal of Functions suggests having a consistent .handle() method instead. This approach has has significant benefits:

It makes the code more immediately understandable across language boundaries by maintaining consistency of supported method signatures
It Better communicates intent
It allows for easier automated code generation or translation
It eases ongoing support burden by minimizing language-specific deviations
It maintains consistent mental models across codebases
It makes it clearer that the object is a "handler" rather than just any callable

Using .handle() is not anti-idiomatic, as many frameworks use this method-based approach rather than making objects callable (e.g., Flask class-based views use .dispatch_request()).

This was the original implementation, and after exploring usage of Python functions as an ASGI callable, it is clear the named handle() method is better overall. This PR reverts to that approach.

[knative-prow]

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