Add ability to provide multiple IP addresses to DNS resolution override

[lpraneis]

Currently, ClientBuilder::resolve() accepts a domain and a single SocketAddr that is used to override DNS resolution for a specified host. It would be useful to allow this override to accept multiple SocketAddrs to allow a user to provide both an IPv4 and an IPv6 address for a host. If both are provided, hyper will use the happy eyeballs algorithm to decide which protocol to connect over. This is specifically helpful for cases where a user might run on both IPv4 and IPv6-only networks and would like to provide a DNS resolution override in both cases for the same hostname.