exchanges: Refactor time handling and other minor improvements (#1948)

* exchanges: Refactor time handling and other minor improvements

- Updated Kraken wrapper to utilise new time handling methods.
- Simplified Kucoin types by removing unnecessary structures and using direct JSON unmarshalling.
- Improved websocket handling in Kucoin to directly parse candlestick data.
- Modified Lbank types to use the new time representation.
- Adjusted Poloniex wrapper and types to utilise the new time handling.
- Updated Yobit types and wrapper to reflect changes in time representation.
- Introduced DateTime type for better handling of specific time formats.
- Added tests for DateTime unmarshalling to ensure correctness.
- Rid UTC().Unix and UTC().UnixMilli as it's not needed
- Correct Huobi timestamp usage for some endpoints.
- Rid RFC3339 time parsing since Go does that automatically.

* exchanges: Refactor JSON unmarshalling for various types and improve test coverage

* linter: Update error message in TestGetKlines

* refactor: Simplify JSON unmarshalling in MovementHistory and improve test assertions in GetKlines

* refactor: Improve JSON unmarshalling for channel name and clarify comment in wsProcessOpenOrders

* refactor: Update time handling in Huobi types to use types.Time for createdAt fields and relax GetLiquidationOrders test

* refactor: Move wsTicker, wsSpread, wsTrades, and wsCandle types to kraken_types.go for better organistion

* refactor: Add validation for underlying parameter in GetExpirationTime and update tests

exchanges/bitmex/bitmex_types.go

@@ -103,7 +103,7 @@ type Execution struct {
 	TimeInForce           string    `json:"timeInForce"`
 	Timestamp             time.Time `json:"timestamp"`
 	TradePublishIndicator string    `json:"tradePublishIndicator"`
-	TransactTime          string    `json:"transactTime"`
+	TransactTime          time.Time `json:"transactTime"`
 	TrdMatchID            string    `json:"trdMatchID"`
 	Triggered             string    `json:"triggered"`
 	UnderlyingLastPx      float64   `json:"underlyingLastPx"`
@@ -187,7 +187,7 @@ type Instrument struct {
 	PrevTotalTurnover              float64   `json:"prevTotalTurnover"`
 	PrevTotalVolume                float64   `json:"prevTotalVolume"`
 	PublishInterval                string    `json:"publishInterval"`
-	PublishTime                    string    `json:"publishTime"`
+	PublishTime                    time.Time `json:"publishTime"`
 	QuoteCurrency                  string    `json:"quoteCurrency"`
 	QuoteToSettleMultiplier        int64     `json:"quoteToSettleMultiplier"`
 	RebalanceInterval              string    `json:"rebalanceInterval"`
@@ -573,7 +573,7 @@ type TransactionInfo struct {
 	TransactID     string    `json:"transactID"`
 	Network        string    `json:"network"`
 	TransactStatus string    `json:"transactStatus"`
-	TransactTime   string    `json:"transactTime"`
+	TransactTime   time.Time `json:"transactTime"`
 	TransactType   string    `json:"transactType"`
 	Tx             string    `json:"tx"`
 }