[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/picker_test.go

@@ -0,0 +1,34 @@
+package client
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestServerPicker_UnavailableServers(t *testing.T) {
+	connectionTimeout = 5 * time.Second
+
+	sp := ServerPicker{
+		TokenStore: nil,
+		PeerID:     "test",
+	}
+	sp.ServerURLs.Store([]string{"rel://dummy1", "rel://dummy2"})
+
+	ctx, cancel := context.WithTimeout(context.Background(), connectionTimeout+1)
+	defer cancel()
+
+	go func() {
+		_, err := sp.PickServer(ctx)
+		if err == nil {
+			t.Error(err)
+		}
+		cancel()
+	}()
+
+	<-ctx.Done()
+	if errors.Is(ctx.Err(), context.DeadlineExceeded) {
+		t.Errorf("PickServer() took too long to complete")
+	}
+}