Add support for CredentialProviderConfig as part of NodeConfig.kubelet.config

**What would you like to be added**:
I want to be able to override the default CredentialProviderConfig settings through NodeConfig. Right now, if you include the configuration in the kubelet config section, it gets written to /etc/kubernetes/kubelet/config.json.d/40-nodeadm.conf. The settings are superseeded by the values in the environment file, /etc/eks/kubelet/environment.

**Why is this needed**:
I want to be able to configure the kubelet to send the service account token bound to the pod for which the image is being pulled to the credential provider plugin. See https://kubernetes.io/docs/tasks/administer-cluster/kubelet-credential-provider/. I want to be able to do this using eksctl's overrideBootstrapCommand which accepts a nodeConfig.

Trace how NodeConfig.kubelet.config is written to /etc/kubernetes/kubelet/config.json.d/40-nodeadm.conf and how /etc/eks/kubelet/environment takes precedence. Verify the credential provider settings can be overridden through eksctl's overrideBootstrapCommand and confirm the resulting kubelet configuration passes the requested service account token setting to the plugin.

Written by the indexing model from the issue text.