The previous use of a static variable for the runtime prevented it from
shutting down cleanly, because shutdown requires dropping or taking
ownership of it. This PR switches the static variable to a handle only,
and introduces a new trait to pass a handle to the async runtime to the
constellation, where it can be shut-down along with other components and
help reduce our count of still running threads after shutdown.
Testing: manual testing, and covered by unit-test in net, and wpt tests.
Fixes: part of - https://github.com/servo/servo/issues/30849
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Signed-off-by: gterzian <2792687+gterzian@users.noreply.github.com>
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