Controller: add a command to fetch HS descriptor from HSdir(s)
Last part of #3521 (moved), this is a bigger one. The ability to ask tor to fetch an HS descriptor from either the default HSdir set/replicas or onto the some specific ones given by the user.
This particular command will trigger external connections to fetch those descriptors.
Furthermore, we should think about if we want the results cached by the client or not which can have bad side effects depending on the intention. Considering #3523 (moved), if tor is able to do that at some point, there could be a use for this command to fetch and store a desc. from a specific HSdir.
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