feat(stackable-versioned): Add K8s specific features

[Techassi]

Part of stackabletech/issues#507

#[versioned(
    version(name = "v1alpha1"),
    version(name = "v1beta1"),
    version(name = "v1"),
    # 👇 new code
    k8s(group = "stackable.tech")
)]
#[derive(Clone, Debug, Deserialize, Serialize, JsonSchema)]
pub struct FooSpec {
    #[versioned(
        added(since = "v1beta1"),
        changed(since = "v1", from_name = "bah", from_type = "u16")
    )]
    bar: usize,
    baz: bool,
}

You can also generate merged CRDs:

let crd = Foo::merged_crd("v1")?;
let yaml = serde_yaml::to_string(&merged_crd)?;

[NickLarsenNZ]

LGTM