GCPMachinePool lacks Alias IP support

[mainak90]

/kind bug

What steps did you take and what happened:

1. Trying to use the CAPI GCP controller to spin up a kubernetes cluster with native routing enabled.
2. Using this unmerged fork: Minimal MachinePool support #1506

What did you expect to happen:

1. Expected the GCPMachinePool CRD to support spec.AliasIPs to allow assigning secondary subnets to Managed Instance Groups in the backend as aliasIPs for native routing.
2. But it seems that the GCPMachinePool spec have aliasIps commented out

   cluster-api-provider-gcp/exp/api/v1beta1/gcpmachinepool_types.go

   Line 51 in 81fd414

   // AliasIPRanges []infrav1.AliasIPRange `json:"aliasIPRanges,omitempty"`

3. The CRD does not have support to add in AliasIPs so there is no way to have a GCPMachinePool resource accept it in spec.
4. AliasIPRanges is currently passed along as an emptyslice to instance.NetworkInterfaces as well here.

Anything else you would like to add:
Can you please add GCPMachinePool to have support to allocate AliasIPs to underlying MIGs.

Environment:

• Cluster-api version: This fork Minimal MachinePool support #1506
• Minikube/KIND version: v1.26.15 and above
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release): Ubuntu-20-04