kopium library pre-publish follow-ups

[clux]

quick notes on things i think can be improved oob;

• make build.rs example deterministic sans/reqwest/tokio Build script examples #380
• make warning header generation (generated by kopium) an opt-in the TypeGenerator (it's hard to find kopium version from a library build.rs) - show flags used instead of cli flags when using lib interface and use it in build.rs
• make unique required vectors such as derive_traits be a BTreeSet to avoid checking for uniqueness on inserting
• try to move TypeGenerator::generate_rust_types_for align with Display (see https://github.com/kube-rs/kopium/pull/376/files#r2393927175)

also note to myself to remember to verify docs.rs output before publishing

[clux]

have merged the example PR which adds one using cli and one using build script, and also makes them runnable in CI.

the remaining things here now are more the remaining nice to have nits that would be good to look at now. they are small nits, but i think at least having a way to provide flag output (in the warning header) would be useful by default.