Before making changes, move router pick address functions to their own C file

As part of Sponsor 55, let's move these relay functions to their own C files:

In src/feature/relay/resolve_addr_relay.c

• router_pick_published_address()

In src/app/config/resolve_addr.c

• resolve_my_address()
• get_last_resolved_addr()
• reset_last_resolved_addr()
• last_resolved_addr
• is_local_addr()
• is_local_addr_impl()

These are relay-level now, but I think they should become client-level:

• router_new_address_suggestion()
• router_guess_address_from_dir_headers()

We should also move any related functions.

And write unit tests :-)