Stream response body in ASGITransport

[jhominal]

Summary

As part of my job, we needed a variant of ASGITransport that
supports streaming (as in encode#2186), and this is my PR to implement that.

Something that I am particularly proud of is that this PR was written without having to spawn a new task, with the consequence that it avoids issues related to task groups and context variables.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.