Add kyaml as supported output format for kubeseal

Which component: kubeseal

Is your feature request related to a problem? Please describe. Kubernetes 1.37 offers stable support for kyaml output of resources; see also https://kubernetes.io/blog/2026/08/26/kubernetes-v1-37-release/#kyaml.

As kyaml is a subset of yaml, it can be used as a full replacement for manifests, with the added benefit of being less ambiguous than yaml, hence potentially preventing issues.

As such it would be nice to allow kyaml output from kubeseal, to allow one to consistently use this for resources in a Kubernetes project and subsequently enforce this with e.g. CI linters.

Describe the solution you'd like Allow passing --format kyaml to kubeseal to have the resources being output in the kyaml format.

Describe alternatives you've considered n/a

Additional context https://kubernetes.io/blog/2026/08/11/how-to-pretty-print-kubernetes-yaml-as-kyaml/