Add Debug extension

[Kludex]

• Closes Proposal: "http.response.template" extension. #135

[Kludex]

Just as a note... I think the issue that motivated this PR was more than just a "debug" extension.

I think it was more about having a namespace on which the application could add any kind of data related to the execution - because right now the ASGI applications use any field that may not conflict with the ASGI spec itself. I'm mentioned this based on the whole issue, but specifically about this comment.

In any case, maybe it's interesting to discuss if we should really add this "debug" extension or just add document a namespace to "execution" information. 🤷

[andrewgodwin]

My recollection of the original motivation (granted, it's been 3 years) was that it was purely for testing - scenarios where you would be passing the raw ASGI response into more code. Server behaviour should be governed by their own extensions (private or spec'd), I think.

[Kludex]

Yeah. This PR already satisfies my needs on Starlette (which is documented on the issue).

[andrewgodwin]

Looks good apart from a single word!

[Kludex]

Applied 🙏

[andrewgodwin]

Lovely, thanks for your work on this!