arti-*: Rename hs feature to onion-service-client

Ian Jacksonrequested to merge
Diziet/arti:hs-naming-arti into main

Rationale as per #756 (closed).

This is the controversial part of !1033 (merged), split off to allow !1033 (merged) to merge quickly.

Will c&p the discussion so far from there.

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