tor-netdoc: Update HsDesc, HsDescOuter to accept the hs_desc_sign cert as an argument.

This will enable us to (eventually) the load the descriptor signing key cert from the keystore (as opposed to always recomputing it when building the HsDesc).

I'm working on a similar change for passing the intro_auth_key_cert and intro_enc_key_cert as fields inside IntroPointDesc (but it's proving a bit tricky, because they're both cross-certified with the descriptor signing key, which is a difficult to retrieve when the IntroPointDescs are created, because the TP is unknown).

Part of #1048 (closed)