Response streaming support for Python runtime

**Note**
> * Please vote on this issue by adding a 👍 reaction to the original issue to help the community and maintainers prioritize this request
> * Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do * not help prioritize the request
> If you are interested in providing additional feedback, please leave a comment

**Description**
Lambda response streaming allows you to incremental stream responses back to clients rather than waiting for the entire response to be buffered first, reducing Time to First Byte. It is currently only supported for Node.js runtime. We would expand support for response streaming to Python runtimes.

**Additional context**
Add additional context here

No implementation files, tests, or entry points are named in the issue. Start by reviewing the existing Node.js response-streaming support and the Python runtime architecture; done means Python runtimes can incrementally stream Lambda responses to clients with equivalent support.

Written by the indexing model from the issue text.