Support IPv6 local_addr for UDP services

[rapiz1]

rathole/src/helper.rs

Lines 40 to 46 in e6dd0c8

	/// Create a UDP socket and connect to `addr`
	pub async fn udp_connect<A: ToSocketAddrs>(addr: A) -> Result<UdpSocket> {
	// FIXME: This only works for IPv4
	let s = UdpSocket::bind("0.0.0.0:0").await?;
	s.connect(addr).await?;
	Ok(s)
	}

UdpSocket::bind("0.0.0.0:0").await? binds to a IPv4 UDP socket and cannot be used to connect to an IPv6 address. A match on addr can solve this issue.