Add option to advertise relay addresses along with other addresses

Currently, if we want to advertise relay addresses, we have to specify all the addresses we want to advertise in the Addresses.Announce config field (because that field correctly replaces all addresses instead of augmenting them). We should have a way to either say "Add relay addresses to my list of addresses that I announce!" or "add this set of addresses to the list of addresses that I announce".
I'm leaning towards the relay specific option, but opinions are welcome.

Start at the Addresses.Announce configuration and the code path that advertises relay addresses. Decide whether the option should add relay addresses specifically or accept an additional address set, then verify that the chosen configuration augments rather than replaces the existing announced addresses.

Written by the indexing model from the issue text.