exchanges/futures: Implement open interest (#1417)

* adds open interest to exchanges

* ADDS TESTING YEAH

* New endpoints, BTSE, RPCS, cached

* slight design change, begin gateio

You will need to get cached for
each exchange that supports it

* gateio, huobi, rpc

* fix up kraken, cache retrieval

* okx, gateio

* finalising all implementations and tests

* definitely my final ever commit on this

* Well, well, well

* final v2

* quick fix of bug

* test coverage, assert notempty, test helper

Added a new testhelper for currency
management because its very annoying
in a parallel test setting which wastes
so much space otherwise

* minimises REST requests for Open Interest

* types.Number merge misses

* Minimises Kraken REST calls

* len change, value -> pointer receiver

* further fixup

* fixes gateio, batch calculates open interest

* single gateio, lint const fixes

* rejig and more thorough oi for huobi

* formatting expansion

* minor fix for handling expiring contracts

* rm unused Binance strings

* add bybit support, fix bybit issues

* oopsie doopsie, dont look at my whoopsie

* Fix issue, remove feature

* move an irrelevant function for the pr

* mini bybit upgrades

* fixes cli request bug

exchanges/exchange_types.go

@@ -179,13 +179,21 @@ type FeaturesSupported struct {
 type FuturesCapabilities struct {
 	FundingRates                    bool
 	MaximumFundingRateHistory       time.Duration
+	FundingRateBatching             map[asset.Item]bool
 	SupportedFundingRateFrequencies map[kline.Interval]bool
 	Positions                       bool
 	OrderManagerPositionTracking    bool
 	Collateral                      bool
 	CollateralMode                  bool
 	Leverage                        bool
-	FundingRateBatching             map[asset.Item]bool
+	OpenInterest                    OpenInterestSupport
+}
+
+// OpenInterestSupport helps breakdown a feature and how it is supported
+type OpenInterestSupport struct {
+	Supported          bool
+	SupportedViaTicker bool
+	SupportsRestBatch  bool
 }
 
 // MarginCapabilities stores the exchange's margin capabilities