feat: Add GenericClusterConfig and add docs on usage with own CC (#606)

**What problem does this PR solve?**:
GenericClusterConfig provides for extension of any CC - very useful!
Added docs on how to add CAREN your own ClusterClass as well so users
can go further than just using our example ClusterClasses.

**Which issue(s) this PR fixes**:
Fixes #

**How Has This Been Tested?**:
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Author: jimmidyson

api/v1alpha1/clusterconfig_types.go

@@ -32,6 +32,8 @@ var (
 	awsClusterConfigCRDDefinition []byte
 	//go:embed crds/caren.nutanix.com_nutanixclusterconfigs.yaml
 	nutanixClusterConfigCRDDefinition []byte
+	//go:embed crds/caren.nutanix.com_genericclusterconfigs.yaml
+	genericClusterConfigCRDDefinition []byte
 
 	dockerClusterConfigVariableSchema = variables.MustSchemaFromCRDYAML(
 		dockerClusterConfigCRDDefinition,
@@ -42,6 +44,9 @@ var (
 	nutanixClusterConfigVariableSchema = variables.MustSchemaFromCRDYAML(
 		nutanixClusterConfigCRDDefinition,
 	)
+	genericClusterConfigVariableSchema = variables.MustSchemaFromCRDYAML(
+		genericClusterConfigCRDDefinition,
+	)
 )
 
 // +kubebuilder:object:root=true
@@ -103,7 +108,7 @@ type DockerClusterConfigSpec struct {
 	ControlPlane *DockerNodeConfigSpec `json:"controlPlane,omitempty"`
 
 	// Extra Subject Alternative Names for the API Server signing cert.
-	// For the Nutanix provider, the following default SANs will always be added:
+	// For the Docker provider, the following default SANs will always be added:
 	// - localhost
 	// - 127.0.0.1
 	// - 0.0.0.0
@@ -150,6 +155,27 @@ type NutanixClusterConfigSpec struct {
 	ExtraAPIServerCertSANs []string `json:"extraAPIServerCertSANs,omitempty"`
 }
 
+// +kubebuilder:object:root=true
+
+// GenericClusterConfig is the Schema for the genericclusterconfigs API.
+type GenericClusterConfig struct {
+	metav1.TypeMeta   `json:",inline"`
+	metav1.ObjectMeta `json:"metadata,omitempty"`
+
+	// +optional
+	Spec GenericClusterConfigSpec `json:"spec,omitempty"`
+
+	// Extra Subject Alternative Names for the API Server signing cert.
+	// +kubebuilder:validation:UniqueItems=true
+	// +kubebuilder:validation:items:Pattern=`^[a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*$`
+	// +optional
+	ExtraAPIServerCertSANs []string `json:"extraAPIServerCertSANs,omitempty"`
+}
+
+func (s GenericClusterConfig) VariableSchema() clusterv1.VariableSchema { //nolint:gocritic,lll // Passed by value for no potential side-effect.
+	return genericClusterConfigVariableSchema
+}
+
 // GenericClusterConfigSpec defines the desired state of GenericClusterConfig.
 type GenericClusterConfigSpec struct {
 	// Sets the Kubernetes image repository used for the KubeadmControlPlane.

api/v1alpha1/crds/caren.nutanix.com_dockerclusterconfigs.yaml

@@ -258,7 +258,7 @@ spec:
               extraAPIServerCertSANs:
                 description: |-
                   Extra Subject Alternative Names for the API Server signing cert.
-                  For the Nutanix provider, the following default SANs will always be added:
+                  For the Docker provider, the following default SANs will always be added:
                   - localhost
                   - 127.0.0.1
                   - 0.0.0.0