[SL FIle] NetworkSubnetIpAddressServiceSL.GetByIPAddress is not reachible from volumeprovider

[depohmel]

in term to have working SL File solution, Volume restored from snapshot need t be authorized for hosts. It's trivial to get nodes IP address in kubernetes and authorize volume for current nodes by ip address. But nodes come and go. The better way is to authorize volume for a subnet.
SL libs already have a code to get a subnet form ip address

ibmcloud-storage-volume-lib/volume-providers/softlayer/backend/network_subnet_ipaddress_service.go

Line 62 in 5364af4

func (ns *NetworkSubnetIpAddressServiceSL) GetByIPAddress(ipAddress *string) (datatypes.Network_Subnet_IpAddress, error) {

but it is not reachable from volume provider clients.
Could you please add this function into github.com/IBM/ibmcloud-storage-volume-lib/provider/utils or something like this?