Add UI to down-/upload cluster templates

[judge-red]

Kindly make it possible to down-/upload cluster templates through the UI. The API already allows it, but the UI (at least in KKP 2.19) does not seem to offer these options. Of course these options should only be available to users who are able to create cluster templates (or maybe better: use the respective API calls) in the first place.

Our users would like to being able to download the templates to store them as IaC, as well as to download-modify-upload them to quickly create variations.

Unfortunately (but understandably) the API delivers and expects the cluster templates in JSON format. YAML would be preferred if that's somehow feasible but I guess there are tools out there that could do the transformation locally.