arti-client: Mark tor-hsservice a dep of onion-service-client feature.

Currently, arti-client doesn't compile with just the features experimental-api and onion-service-client enabled, because it depends on tor-hsservice in that crate, but that dependency doesn't exist.

While ideally we would be able to tell cargo that's the case, it doesn't seem like that specificity exists, so we should just put tor-hsservice as a dependency of one of experimental-api and onion-service-client. I've chosen onion-service-client since that seem more sensible to me, but either one would work (or perhaps there's some more complex way of poking cargo here that I'm unaware of)

Fixes: #1638