[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/env_generic.go

@@ -0,0 +1,23 @@
+//go:build !linux && !windows && !android
+
+package net
+
+// Init initializes the network environment (no-op on non-Linux/Windows platforms)
+func Init() {
+	// No-op on non-Linux/Windows platforms
+}
+
+// AdvancedRouting returns false on non-Linux/Windows platforms
+func AdvancedRouting() bool {
+	return false
+}
+
+// SetVPNInterfaceName is a no-op on non-Windows platforms
+func SetVPNInterfaceName(name string) {
+	// No-op on non-Windows platforms
+}
+
+// GetVPNInterfaceName returns empty string on non-Windows platforms
+func GetVPNInterfaceName() string {
+	return ""
+}