✨ Add tpl and include functions from Helm

[sathieu]

What this PR does / why we need it:

We use metal3 provider and the image depends on various factors:

• what is the distro (debian/ubuntu/...)
• is the image using 4k or 512-bytes block size ?
• which k8s version
• ...

Instead of hardcoding the image template in the clusterclass, we need to construct it from various other variables.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

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[k8s-ci-robot]

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[sathieu]

/area clusterclass

[fabriziopandini]

Allows to evaluate strings as templates inside a template

We implemented https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/proposals/20220330-topology-mutation-hook.md which provides a better alternative for when you are hitting limitations of json patches.

Notably, the benefits that you will get from moving patches to an external component are way more that addressing this specific use case.

[sathieu]

There is currently only one provider available (https://cluster-api.sigs.k8s.io/reference/providers#runtime-extensions), nutanix, which works like a black box. The effective changes made by the provider are not detailed (unless you read the code). I find this confusing.

I agree that JSON patches are limited, but this feature (more dynamic patches) should be in core IMO. Maybe another method is needed?

Kubernetes is including CEL mutations (currently beta, off-by-default), could this be a part forward in core ?

Also, giving access to intermediary resources would be great (to understand what is changed by each step). This is another topic.

[sbueringer]

I think we should not try to build an extensive templating engine within the core CAPI controller.

If there is demand I would recommend building that out-of-tree, while also not trying to reinvent the wheel considering numerous pre-existing projects and languages for this