New function (on NetDirProvider? or extension trait?) to block until a netdir is available
See discussion at !1465 (comment 2928347) : it's probably useful sometimes to have an async function that either returns a timely netdir, or blocks until we have one.
(This is especially handy for code that wants to act sensibly while waiting for re-bootstrap.)
I'm adding one to tor-hsservice as part of my intro-points work, but it's likely it should move elsewhere.