Turns out that passing client authorization keys to ADD_ONION for v3 was
not working because we were not setting the "is_client_auth_enabled"
flag to true once the clients were configured. This lead to the
descriptor being encoded without the clients.

This patch removes that flag and instead adds an inline function that
can be used to check if a given service has client authorization
enabled.

This will be much less error prone of needing to keep in sync the client
list and a flag instead.

Fixes #40378



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