[#168] accounting: Implement binary/JSON encoders/decoders on Decimal

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

v2/accounting/json_test.go

@@ -0,0 +1,20 @@
+package accounting_test
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/accounting"
+	"github.com/stretchr/testify/require"
+)
+
+func TestDecimalJSON(t *testing.T) {
+	i := generateDecimal(10)
+
+	data, err := i.MarshalJSON()
+	require.NoError(t, err)
+
+	i2 := new(accounting.Decimal)
+	require.NoError(t, i2.UnmarshalJSON(data))
+
+	require.Equal(t, i, i2)
+}