Support creating multicast netlink connection

[cathay4t]

Introduced new_multicast_connection() and
new_multicast_connection_with_socket() taking &[MulticastGroup] as
argument.

Example code:

let (conn, mut _handle, mut messages) = new_multicast_connection(&[
    MulticastGroup::Link,
]).unwrap()

// Spawn `Connection` to start polling netlink socket.
tokio::spawn(conn);

// Start receiving events through `messages` channel.
while let Some((message, _)) = messages.next().await {
    let payload = message.payload;
    println!("{payload:?}");
}

Example code updated.