[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/relay/client/conn.go

@@ -0,0 +1,74 @@
+package client
+
+import (
+	"net"
+	"time"
+
+	"github.com/netbirdio/netbird/relay/messages"
+)
+
+// Conn represent a connection to a relayed remote peer.
+type Conn struct {
+	client      *Client
+	dstID       messages.PeerID
+	messageChan chan Msg
+	instanceURL *RelayAddr
+}
+
+// NewConn creates a new connection to a relayed remote peer.
+// client: the client instance, it used to send messages to the destination peer
+// dstID: the destination peer ID
+// messageChan: the channel where the messages will be received
+// instanceURL: the relay instance URL, it used to get the proper server instance address for the remote peer
+func NewConn(client *Client, dstID messages.PeerID, messageChan chan Msg, instanceURL *RelayAddr) *Conn {
+	c := &Conn{
+		client:      client,
+		dstID:       dstID,
+		messageChan: messageChan,
+		instanceURL: instanceURL,
+	}
+
+	return c
+}
+
+func (c *Conn) Write(p []byte) (n int, err error) {
+	return c.client.writeTo(c, c.dstID, p)
+}
+
+func (c *Conn) Read(b []byte) (n int, err error) {
+	msg, ok := <-c.messageChan
+	if !ok {
+		return 0, net.ErrClosed
+	}
+
+	n = copy(b, msg.Payload)
+	msg.Free()
+	return n, nil
+}
+
+func (c *Conn) Close() error {
+	return c.client.closeConn(c, c.dstID)
+}
+
+func (c *Conn) LocalAddr() net.Addr {
+	return c.client.relayConn.LocalAddr()
+}
+
+func (c *Conn) RemoteAddr() net.Addr {
+	return c.instanceURL
+}
+
+func (c *Conn) SetDeadline(t time.Time) error {
+	//TODO implement me
+	panic("SetDeadline is not implemented")
+}
+
+func (c *Conn) SetReadDeadline(t time.Time) error {
+	//TODO implement me
+	panic("SetReadDeadline is not implemented")
+}
+
+func (c *Conn) SetWriteDeadline(t time.Time) error {
+	//TODO implement me
+	panic("SetReadDeadline is not implemented")
+}