tor-hsservice: Give the descriptor publisher a separate launch function.

gabi-250requested to merge
gabi-250/arti:publisher-launch-fn into main

This also makes Publisher::new synchronous.

If Reactor::new fails, Publisher::launch propagates the error to its caller (to achieve this, I had to give PublisherError a new ReactorLaunch variant and make ReactorError and UploadError crate-public).

Closes #1042 (closed)

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