Expose KubeAdm apiserver extra-args configuration in EKS-A manifest

**What would you like to be added**:

Expose KubeAdm apiserver extra-args configuration in EKS-A manifest

**Why is this needed**:

KubeAdm cluster-api provider allows users to customize kube-apiserver arguments for configuring additional capabilities or experimental feature flags via extra-args (as seen in [kubeadm_types.go](https://github.com/kubernetes-sigs/cluster-api/blob/main/bootstrap/kubeadm/api/v1beta1/kubeadm_types.go)). This helps one to configure custom webhook based identity implementations. Given that eks-a wraps around kubeadm controlplane, it would be useful to provide a "extraArgs" at eks-a cluster manifest level that will propagate the arguments to kubeadm control plane config

Sample of the spec in kubeadmConfig in Cluster-API

```
spec:
infrastructureTemplate:
apiVersion: infrastructure.cluster.x-k8s.io/v1alpha3
kind: AWSMachineTemplate
name: capi-etcd-control-plane
kubeadmConfigSpec:
clusterConfiguration:
apiServer:
extraArgs:
cloud-provider: aws
```

Start with the linked kubeadm_types.go definition to understand the expected apiServer.extraArgs shape, then locate the EKS-A manifest and the code that maps it into the kubeadm control-plane configuration. Done means an extraArgs setting is accepted at the EKS-A manifest level and propagated to the kubeadm API server configuration.

Written by the indexing model from the issue text.