CancelAllOrders should return an array of objects

[It currently returns only an error](https://github.com/alpacahq/alpaca-trade-api-go/blob/7d20c8f30eaaf64619c4bd23b9855dd04dcccd35/alpaca/rest.go#L651-L663) but the API returns an array object that includes the order ID and HTTP status code for each status request. See https://docs.alpaca.markets/reference/deleteallorders-1.

There was a very similar issue with `CloseAllPositions` that was fixed in #282 that could be used as an example for resolving this issue.