[client] Use native windows sock opts to avoid routing loops (#4314)

- Move `util/grpc` and `util/net` to `client` so `internal` packages can be accessed
 - Add methods to return the next best interface after the NetBird interface.
- Use `IP_UNICAST_IF` sock opt to force the outgoing interface for the NetBird `net.Dialer` and `net.ListenerConfig` to avoid routing loops. The interface is picked by the new route lookup method.
- Some refactoring to avoid import cycles
- Old behavior is available through `NB_USE_LEGACY_ROUTING=true` env var

client/net/dialer_init_linux.go

@@ -0,0 +1,12 @@
+//go:build !android
+
+package net
+
+import "syscall"
+
+// init configures the net.Dialer Control function to set the fwmark on the socket
+func (d *Dialer) init() {
+	d.Dialer.Control = func(_, _ string, c syscall.RawConn) error {
+		return setRawSocketMark(c)
+	}
+}