Enable setting and resetting a thread-local parent header

[jasongrout]

As mentioned in #1289 (comment) and #1451, there should be a public api for setting and clearing a thread-local parent header. This PR creates a recommended public api for setting/resetting a single thread's parent without setting the global fallback header. An ipywidget OutputWidget can use this to temporarily redirect output from a single thread, for example.

Changelog entry:

Output from threads can be explicitly routed using the new get_ipython().set_thread_parent(), which also returns a token that can be used to undo the set with get_ipython().reset_thread_parent(token). Unlike set_parent(), using set_thread_parent() does not affect the default shell parent, so does not affect output routing in other threads. Since the thread parent is stored in a thread ContextVar, techniques for propagating a thread's ContextVars will also propagate the thread parent without having to explicitly call set_thread_parent().

I also added a number of x.parent = ... parent setters to be more consistent - I'm not directly going to use those, but happy to remove those if there is an objection.

Claude Fable 5 assisted in this PR

[jasongrout]

@minrk, @krassowski - this finishes out the ideas from #1289 and #1451 to make it easy to set and reset just the thread parent. This will make the OutputWidget's temporary overriding the parent much easier (i.e., helping fix jupyter-widgets/ipywidgets#4020 more cleanly than jupyter-widgets/ipywidgets#4021). I think both of you have the context from #1289 and #1451 to review this?