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

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

v2/accounting/json.go

@@ -0,0 +1,26 @@
+package accounting
+
+import (
+	accounting "github.com/nspcc-dev/neofs-api-go/v2/accounting/grpc"
+	"google.golang.org/protobuf/encoding/protojson"
+)
+
+func (d *Decimal) MarshalJSON() ([]byte, error) {
+	return protojson.MarshalOptions{
+		EmitUnpopulated: true,
+	}.Marshal(
+		DecimalToGRPCMessage(d),
+	)
+}
+
+func (d *Decimal) UnmarshalJSON(data []byte) error {
+	msg := new(accounting.Decimal)
+
+	if err := protojson.Unmarshal(data, msg); err != nil {
+		return err
+	}
+
+	*d = *DecimalFromGRPCMessage(msg)
+
+	return nil
+}