optionable 0.12.0

Derive macro (and associated marker trait) to derive nested structs/enums with all subfields being optional (e.g. for patches or Kubernetes server side apply).
Documentation 
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fn main() {
    let enabled_versions = (30..=34)
        .map(|v| format!("V1_{v}"))
        .filter(|v| std::env::var(format!("CARGO_FEATURE_K8S_OPENAPI_{v}")).is_ok())
        .collect::<std::collections::BTreeSet<_>>();
    assert!(
        enabled_versions.len() <= 1,
        "\nOnly one `k8s_openapi_*` feature may be enabled at a given time, found: {enabled_versions:?}`\n"
    );
}