Multi template selection in ClusterClass (non-breaking)

[holmesb]

Hey,

Great work on your localStorage feature. This PR adds it to ClusterClasses via new optional topology variables, eg:

variables:
  - name: templateSelector
    value:
      matchTags: ["capmox","ubuntu-24.04","k8s-1.33"]
  - name: localStorage
     value: true

When defined per above, ProxmoxMachineTemplates get localStorage added and set to true. Also templateSelector is added, and sourceNode & templateID are removed.

Without a templateSelector or localStorage defined, behaviour is same as now: PMTs keep explicit fields patched from cloneSpec.machineSpec.*.{sourceNode,templateID}.

No controller code changes are required for this PR; it’s purely ClusterClass templates + docs.

Users following the current examples (with sourceNode/templateID) don’t need to change anything.

I’ve added/updated docs/advanced-setups.md with a short section explaining how to use templateSelector & localStorage when defining a class-powered cluster.

I’ve opened this as a Draft to make it easy for you to review and iterate without pressure. If you’re comfortable, feel free to merge into your PR branch now so reviewers can test the end-to-end ClusterClass flow from a single branch. If you prefer to keep it separate until your main PR lands in ionos-cloud, can leave it Draft and rebase after your merge - your call.

Relates to issue ionos-cloud/capmox ionos-cloud#575 - ClusterClass templateSelector (keeps explicit mode as default).