add SecondaryStorageOption to cluster and workerpool operations

[z0za]

this PR extends the cluster and workerpool operations with SecondaryStorageOption https://containers.cloud.ibm.com/global/swagger-global-api/#/v2/getWorkerPool https://containers.cloud.ibm.com/global/swagger-global-api/#/v2/vpcCreateCluster https://containers.cloud.ibm.com/global/swagger-global-api/#/v2/vpcCreateWorkerPool

tested with the below examples:

CreateClusterV2 % go run main.go -vpcid <vpcid> -subnetid <subnetid> -secondarystorage 900gb.5iops-tier -zone <zoneid> -name zoza-test 
...
Cluster create request was successful
Response: {<clusterid>}

---------------------------

GetWorkerpoolV2 % go run main.go -cluster <clusterid> -workerpool default
...
Get workerpool request was successful
Response: {"flavor":"bx2.4x16","id":"<workerpoolid>","isolation":"public","labels":{"ibm-cloud.kubernetes.io/worker-pool-id":"<workerpoolid>"},"operatingSystem":"UBUNTU_18_64","lifecycle":{"actualState":"","desiredState":"active"},"vpcID":"<vpcid>","workerCount":1,"poolName":"default","provider":"vpc-gen2","zones":[{"id":"<zoneid>","workerCount":1,"subnets":[{"id":"<subnetid>","primary":true}]}],"secondaryStorageOption":{"name":"900gb.5iops-tier","Count":1,"Size":900,"DeviceType":"BLOCK","RAIDConfiguration":"none","profile":"5iops-tier"}}