Add support for placement groups

[Tomy2e]

Add the possibility of attaching an existing Instance Placement Group to a ScalewayMachine.

We should be able to reference the placement group by ID or name.

The API of the field should look like this:

// +kubebuilder:validation:XValidation:rule="(has(self.id) ? 1 : 0) + (has(self.name) ? 1 : 0) == 1",message="exactly one of id or name must be set"
type PlacementGroupSpec struct {
	// ID of the placement group.
	ID *string `json:"id,omitempty"`
	// Name of the placement group.
	Name *string `json:"name,omitempty"`
}

Placement groups managed at the ScalewayCluster level may be implemented later.