Add parsing validations for SnowMachineConfig and SnowDatacenterConfig

Add validations for Snow specific objects (similar to [this ](https://github.com/aws/eks-anywhere/blob/main/pkg/api/v1alpha1/cluster.go#L269)for the Cluster object) to ensure that only valid fields are passed in.

Passing in invalid fields could lead to unexpected behavior like [this](https://github.com/aws/eks-anywhere/issues/5566)

Start with pkg/api/v1alpha1/cluster.go, especially the Cluster validation referenced in the issue, then locate SnowMachineConfig and SnowDatacenterConfig and their validation entry points. Done means invalid or unknown fields are rejected for both Snow-specific objects, consistently with Cluster validation, with coverage for the affected cases.

Written by the indexing model from the issue text.