service/src/lib: Update authority discovery construction

https://github.com/paritytech/substrate/pull/6760 introduces the concept
of an authority discovery `Service` allowing one to communicate with an
authority discovery `Worker`, e.g. to learn the `Multiaddr`s for a given
`AuthorityId`.

Along with the new `Service` structure it also alters the authority
discovery constructor to return both a worker and a service. This
commit adjusts the callside of the constructor, ignoring the `Service`
for now.