Add optional snake_case conversion of member names

[clux]

Using heck as it's the most popular libary out there (plus it's maintained by boats).

It now avoids awkward behaviour of ToSnakeCase not being a true inverse of #[serde(rename_all = "camelCase")] by adding noisy #[serde(rename = "originalName")] on fields that apply. It will be noisy, but only on fields with LONG names.

I think it's generally OK. Perhaps even good enough for -z to be the default, but will wait a bit to see.

Produces code looking like:

#[derive(Serialize, Deserialize, Clone, Debug)]
pub struct ServerPodSelector {
    #[serde(rename = "matchExpressions")]
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub match_expressions: Option<Vec<ServerPodSelectorMatchExpressions>>,
    #[serde(rename = "matchLabels")]
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub match_labels: Option<BTreeMap<String, serde_json::Value>>,
}