[#233] Implement data audit result type with basic methods

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

v2/audit/json_test.go

@@ -0,0 +1,20 @@
+package audit_test
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/audit"
+	"github.com/stretchr/testify/require"
+)
+
+func TestDataAuditResultJSON(t *testing.T) {
+	a := generateDataAuditResult()
+
+	data, err := a.MarshalJSON()
+	require.NoError(t, err)
+
+	a2 := new(audit.DataAuditResult)
+	require.NoError(t, a2.UnmarshalJSON(data))
+
+	require.Equal(t, a, a2)
+}