Provide an example `ClusterClass` with jsonPatch in-place, covered in e2e.

[Danil-Grigorev]

Describe an example cluster-class patch to use in documentation, in order to deploy fleet with agent-initiated installation procedure.

Add e2e test to cover its usage.

Approximately the content should look like:

    - name: fleet-agent-install
      definitions:
        - selector:
            apiVersion: controlplane.cluster.x-k8s.io/v1beta1
            kind: RKE2ControlPlaneTemplate
            matchResources:
              controlPlane: true
          jsonPatches:
            - op: add
              path: /spec/template/spec/files/-
              valueFrom:
                template: |
                    contentFrom:
                      secret:
                        key: pod
                        name: "{{ .builtin.cluster.name }}-agent-install"
                    owner: root:root
                    path: /etc/kubernetes/fleet-install.yaml
                    permissions: "0644"
            - op: add
              path: /spec/template/spec/files/-
              valueFrom:
                template: |
                    contentFrom:
                      secret:
                        key: values
                        name: "{{ .builtin.cluster.name }}-agent-install"
                    owner: root:root
                    path: /etc/kubernetes/fleet-values.yaml
                    permissions: "0644"