This commit extract most of the code that dirclient.c had to handle the end of
a descriptor directory requests (fetch). It is moved into hs_client.c in order
to have one single point of entry and the rest is fully handled by the HS
subsystem.

As part of #30382, depending on how the descriptor ended up stored (decoded or
not), different SOCKS error code can be returned.

Signed-off-by: David Goulet <dgoulet@torproject.org>