Add ConnectAsync support to CreateContextForMultiPortPath.

This change modifies CreateContextForMultiPortPath to use
ConnectAndConfigureSocket and run the context observer asynchronously.

Bug: 1295460, b/271271863
Change-Id: I5d33eed61984d5d8dd7e8b127f34f2d57102a11a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4500184


Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Liza Burakova <liza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148287}