[ip] Rely on `SocketAddr` `enum` Rather Than `SocketAddrV4` `struct`

[ppenna]

Description

Catnip uses the ~~Ipv4Endpoint~~ SocketAddrV4 structure to refer to remote and local endpoints. While this is fine when supporting IPv4 addresses only, once we add support to IPv6 it is make the stack a mess. Therefore, we should change the stack so as to refer to local and remote endpoints using a more generic type, like ~~IpEndpoint~~SocketAddr.