Set user data vtable when creating grpc_lb_addresses.

src/core/ext/client_config/lb_policy_factory.c

src/core/ext/client_config/lb_policy_factory.h

src/core/ext/client_config/resolver_result.c

src/core/ext/lb_policy/grpclb/grpclb.c

src/core/ext/resolver/dns/native/dns_resolver.c

src/core/ext/resolver/sockaddr/sockaddr_resolver.c

test/core/end2end/fake_resolver.c