Support Reservation Affinity in GCPMachine and GCPMachineTemplate

/kind feature **Describe the solution you'd like** Currently, GCPMachine and GCPMachineTemplate do not appear to expose a field to configure Reservation Affinity. Google Compute Engine allows instances to target specific reservations to ensure capacity guarantees or to utilize committed resources (Sole-tenant nodes or zonal reservations). Without this field, CAPG-provisioned nodes rely on the default behavior (often ANY_RESERVATION), which may not be sufficient for users who need to target specific reservations by name or explicitly opt-out of consuming reservations.