Estimate an array of Operations async

[simonmcl]

Operations are estimated individually when using policy: .estimate. When dealing with an array of operations where operation 2 depends on operation 1, estimation will fail to return the correct values, as the second operation will fail.

It would be useful to have a standalone method to estimate bigger operations in an async manner. Something like:

tezosNodeClient.feeEstimator.estimate(operations: ops) { result in
    switch result {
        case .success(let operations):
            // do something with new operations

        case .failure(let error):
            print("Error: \(error)")
    }
}