/kind feature

Describe the solution you'd like
[A clear and concise description of what you want to happen.]

Implement GCPMachinePool using Managed Instance Groups. The GCPMachinePool implementation should follow the same package / group structure as CAPI and be added to the exp package, the experimental feature package. The feature should be gated using the MachinePool flag used in CAPI.

See also: kubernetes-sigs/cluster-api-provider-azure#483 and CAPI Machine Pool Proposal

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Note: As noted below in comments, the kubernetes-sigs/cluster-api#7107 also be supported/implemented as part of this