Feature/subaccount service (#288)

* add TransferToSubAccountService

* fix type

* remove test method

Co-authored-by: adshao <tjusgj@gmail.com>

v2/client.go

@@ -653,6 +653,11 @@ func (c *Client) NewDustTransferService() *DustTransferService {
 	return &DustTransferService{c: c}
 }
 
+// NewTransferToSubAccountService transfer to subaccount service
+func (c *Client) NewTransferToSubAccountService() *TransferToSubAccountService {
+	return &TransferToSubAccountService{c: c}
+}
+
 // NewAssetDividendService init the asset dividend list service
 func (c *Client) NewAssetDividendService() *AssetDividendService {
 	return &AssetDividendService{c: c}

v2/subaccount_service.go

@@ -0,0 +1,70 @@
+package binance
+
+import (
+	"context"
+	"encoding/json"
+)
+
+// TransferToSubAccountService transfer to subaccount
+type TransferToSubAccountService struct {
+	c       *Client
+	toEmail string
+	asset   string
+	amount  string
+}
+
+// ToEmail set toEmail
+func (s *TransferToSubAccountService) ToEmail(toEmail string) *TransferToSubAccountService {
+	s.toEmail = toEmail
+	return s
+}
+
+// Asset set asset
+func (s *TransferToSubAccountService) Asset(asset string) *TransferToSubAccountService {
+	s.asset = asset
+	return s
+}
+
+// Amount set amount
+func (s *TransferToSubAccountService) Amount(amount string) *TransferToSubAccountService {
+	s.amount = amount
+	return s
+}
+
+func (s *TransferToSubAccountService) transferToSubaccount(ctx context.Context, endpoint string, opts ...RequestOption) (data []byte, err error) {
+	r := &request{
+		method:   "POST",
+		endpoint: endpoint,
+		secType:  secTypeSigned,
+	}
+	m := params{
+		"toEmail": s.toEmail,
+		"asset":   s.asset,
+		"amount":  s.amount,
+	}
+	r.setParams(m)
+	data, err = s.c.callAPI(ctx, r, opts...)
+	if err != nil {
+		return []byte{}, err
+	}
+	return data, nil
+}
+
+// Do send request
+func (s *TransferToSubAccountService) Do(ctx context.Context, opts ...RequestOption) (res *TransferToSubAccountResponse, err error) {
+	data, err := s.transferToSubaccount(ctx, "/sapi/v1/sub-account/transfer/subToSub", opts...)
+	if err != nil {
+		return nil, err
+	}
+	res = &TransferToSubAccountResponse{}
+	err = json.Unmarshal(data, res)
+	if err != nil {
+		return nil, err
+	}
+	return res, nil
+}
+
+// TransferToSubAccountResponse define transfer to subaccount response
+type TransferToSubAccountResponse struct {
+	TxnID int64 `json:"txnId"`
+}