[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/hooks/hooks.go

@@ -0,0 +1,93 @@
+package hooks
+
+import (
+	"net/netip"
+	"slices"
+	"sync"
+
+	"github.com/google/uuid"
+)
+
+// ConnectionID provides a globally unique identifier for network connections.
+// It's used to track connections throughout their lifecycle so the close hook can correlate with the dial hook.
+type ConnectionID string
+
+// GenerateConnID generates a unique identifier for each connection.
+func GenerateConnID() ConnectionID {
+	return ConnectionID(uuid.NewString())
+}
+
+type WriteHookFunc func(connID ConnectionID, prefix netip.Prefix) error
+type CloseHookFunc func(connID ConnectionID) error
+type AddressRemoveHookFunc func(connID ConnectionID, prefix netip.Prefix) error
+
+var (
+	hooksMutex sync.RWMutex
+
+	writeHooks         []WriteHookFunc
+	closeHooks         []CloseHookFunc
+	addressRemoveHooks []AddressRemoveHookFunc
+)
+
+// AddWriteHook allows adding a new hook to be executed before writing/dialing.
+func AddWriteHook(hook WriteHookFunc) {
+	hooksMutex.Lock()
+	defer hooksMutex.Unlock()
+	writeHooks = append(writeHooks, hook)
+}
+
+// AddCloseHook allows adding a new hook to be executed on connection close.
+func AddCloseHook(hook CloseHookFunc) {
+	hooksMutex.Lock()
+	defer hooksMutex.Unlock()
+	closeHooks = append(closeHooks, hook)
+}
+
+// RemoveWriteHooks removes all write hooks.
+func RemoveWriteHooks() {
+	hooksMutex.Lock()
+	defer hooksMutex.Unlock()
+	writeHooks = nil
+}
+
+// RemoveCloseHooks removes all close hooks.
+func RemoveCloseHooks() {
+	hooksMutex.Lock()
+	defer hooksMutex.Unlock()
+	closeHooks = nil
+}
+
+// AddAddressRemoveHook allows adding a new hook to be executed when an address is removed.
+func AddAddressRemoveHook(hook AddressRemoveHookFunc) {
+	hooksMutex.Lock()
+	defer hooksMutex.Unlock()
+	addressRemoveHooks = append(addressRemoveHooks, hook)
+}
+
+// RemoveAddressRemoveHooks removes all listener address hooks.
+func RemoveAddressRemoveHooks() {
+	hooksMutex.Lock()
+	defer hooksMutex.Unlock()
+	addressRemoveHooks = nil
+}
+
+// GetWriteHooks returns a copy of the current write hooks.
+func GetWriteHooks() []WriteHookFunc {
+	hooksMutex.RLock()
+	defer hooksMutex.RUnlock()
+	return slices.Clone(writeHooks)
+}
+
+// GetCloseHooks returns a copy of the current close hooks.
+func GetCloseHooks() []CloseHookFunc {
+	hooksMutex.RLock()
+	defer hooksMutex.RUnlock()
+	return slices.Clone(closeHooks)
+}
+
+// GetAddressRemoveHooks returns a copy of the current listener address remove hooks.
+func GetAddressRemoveHooks() []AddressRemoveHookFunc {
+	hooksMutex.RLock()
+	defer hooksMutex.RUnlock()
+	return slices.Clone(addressRemoveHooks)
+}