fix: httpx.Client is slow when sending large data

[xiexianbin]

This commit addresses an issue where sending large data with httpx.Client was extremely slow. The problem was that the entire data payload was being loaded into memory and sent in a single operation.

This fix refactors the sending process for iterable bytes types to use a chunked approach with yield, which significantly improves performance and reduces memory usage for large payloads.

Summary

pip install httpx==0.27.0

test case ref https://github.com/encode/httpcore/issues/1029, close issue https://github.com/encode/httpx/issues/3660

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] 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.