Thread Context should collect unique response headers in async actions #19222
Description
The current implementation of the ThreadContext (#17804) works by having the context passed from thread-to-thread, and the last thread to write to it wins. In the case of async (e.g., scatter/gather) request/response schemes, this won't work if anything unique happens along the way.
This covers all current needs, but it could prove to be very useful to have it actually collect unique async response headers.