Implement Webhook Validation for GCPMachinePool

As noted in the review of PR #1506, `GCPMachinePool` currently lacks comprehensive validation for its spec fields. We need to implement `ValidateCreate` and `ValidateUpdate` webhooks to enforce field constraints (e.g., ensuring required fields are present, validating enums). Where possible, we should share logic with the existing `GCPMachine` validation. Relevant discussion: https://github.com/kubernetes-sigs/cluster-api-provider-gcp/pull/1506#discussion_r2681860031