[misc] Move shared components to shared directory (#4286)

Moved the following directories:

```
  - management/client → shared/management/client
  - management/domain → shared/management/domain
  - management/proto → shared/management/proto
  - signal/client → shared/signal/client
  - signal/proto → shared/signal/proto
  - relay/client → shared/relay/client
  - relay/auth → shared/relay/auth
```

and adjusted import paths

shared/signal/client/client.go

@@ -0,0 +1,77 @@
+package client
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"strings"
+
+	"github.com/netbirdio/netbird/shared/signal/proto"
+	"github.com/netbirdio/netbird/version"
+
+	"golang.zx2c4.com/wireguard/wgctrl/wgtypes"
+)
+
+// A set of tools to exchange connection details (Wireguard endpoints) with the remote peer.
+
+// Status is the status of the client
+type Status string
+
+const StreamConnected Status = "Connected"
+const StreamDisconnected Status = "Disconnected"
+
+const (
+	// DirectCheck indicates support to direct mode checks
+	DirectCheck uint32 = 1
+)
+
+type Client interface {
+	io.Closer
+	StreamConnected() bool
+	GetStatus() Status
+	Receive(ctx context.Context, msgHandler func(msg *proto.Message) error) error
+	Ready() bool
+	IsHealthy() bool
+	WaitStreamConnected()
+	SendToStream(msg *proto.EncryptedMessage) error
+	Send(msg *proto.Message) error
+	SetOnReconnectedListener(func())
+}
+
+// UnMarshalCredential parses the credentials from the message and returns a Credential instance
+func UnMarshalCredential(msg *proto.Message) (*Credential, error) {
+
+	credential := strings.Split(msg.GetBody().GetPayload(), ":")
+	if len(credential) != 2 {
+		return nil, fmt.Errorf("error parsing message body %s", msg.Body)
+	}
+	return &Credential{
+		UFrag: credential[0],
+		Pwd:   credential[1],
+	}, nil
+}
+
+// MarshalCredential marshal a Credential instance and returns a Message object
+func MarshalCredential(myKey wgtypes.Key, myPort int, remoteKey string, credential *Credential, t proto.Body_Type, rosenpassPubKey []byte, rosenpassAddr string, relaySrvAddress string) (*proto.Message, error) {
+	return &proto.Message{
+		Key:       myKey.PublicKey().String(),
+		RemoteKey: remoteKey,
+		Body: &proto.Body{
+			Type:           t,
+			Payload:        fmt.Sprintf("%s:%s", credential.UFrag, credential.Pwd),
+			WgListenPort:   uint32(myPort),
+			NetBirdVersion: version.NetbirdVersion(),
+			RosenpassConfig: &proto.RosenpassConfig{
+				RosenpassPubKey:     rosenpassPubKey,
+				RosenpassServerAddr: rosenpassAddr,
+			},
+			RelayServerAddress: relaySrvAddress,
+		},
+	}, nil
+}
+
+// Credential is an instance of a GrpcClient's Credential
+type Credential struct {
+	UFrag string
+	Pwd   string
+}