[#172] v2/netmap: Add JSON converter for node info

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

v2/netmap/json_test.go

@@ -0,0 +1,22 @@
+package netmap_test
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/netmap"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNodeInfoJSON(t *testing.T) {
+	exp := generateNodeInfo("public key", "/multi/addr", 2)
+
+	t.Run("non empty", func(t *testing.T) {
+		data := netmap.NodeInfoToJSON(exp)
+		require.NotNil(t, data)
+
+		got := netmap.NodeInfoFromJSON(data)
+		require.NotNil(t, got)
+
+		require.Equal(t, exp, got)
+	})
+}