Add MicrovmClusterTemplate to support ClusterClass

**Describe the solution you'd like:**
To enable support for [ClusterClass](https://cluster-api.sigs.k8s.io/tasks/experimental-features/cluster-classes.html) we need to add a new API type called `MicrovmClusterTemplate` with associated webhook.

For an example see the AWS provider [here](https://github.com/kubernetes-sigs/cluster-api-provider-aws/blob/1a61dfc39649749b8b8dfad8334909a388d675a7/api/v1beta1/awsclustertemplate_types.go) and [here](https://github.com/kubernetes-sigs/cluster-api-provider-aws/blob/1a61dfc39649749b8b8dfad8334909a388d675a7/api/v1beta1/awsclustertemplate_webhook.go)

**Why do you want this feature:**

**Anything else you would like to add:**
[Miscellaneous information that will assist in solving the issue.]

Start by reviewing the AWS provider examples linked in the issue: awsclustertemplate_types.go and awsclustertemplate_webhook.go. Add the MicrovmClusterTemplate API type and its associated webhook so the provider supports ClusterClass; the issue does not name project tests to run.

Written by the indexing model from the issue text.