FTX: Add REST staking and basic subaccount functionality (#692)

* Add FTX staking, missing margin APIs and basic subaccount support

* Fix backtester tests and add optional subaccount support to exchange_wrapper_issues tool

* subAccount to subaccount

* Fix TyPo

* Expand test coverage

* Address nitterinos

* Fix typos

* Remove unusued error type

exchanges/exchange_types.go

@@ -184,10 +184,11 @@ type API struct {
 	Endpoints *Endpoints
 
 	Credentials struct {
-		Key      string
-		Secret   string
-		ClientID string
-		PEMKey   string
+		Key        string
+		Secret     string
+		ClientID   string
+		PEMKey     string
+		Subaccount string
 	}
 
 	CredentialsValidator struct {