refactor(model, util, validate)!: Convert CommandOptionChoice to a struct with value variants

[suneettipirneni]

This PR refactors CommandOptionChoice to be a struct rather than an enum. The main reasoning behind this has to do with the fact that all choice variants have fields in common (name and name_localizations). Accessing these fields currently requires the user to exhaustively match an enum variant. With these changes, the shared fields can be accessed directly and instead variance is introduced by using an enum type on the value field.

[zeylahellyer]

Could be nice to have this for name_localizations as well?

[suneettipirneni]

I was thinking about my implementation and think it can be improved. What are your thoughts on a breaking change to make CommandOptionChoice a struct and have it's value be an enum. ie

struct CommandOptionChoice {
	pub name: String,
	pub name_localizations: Option<HashMap<String, String>>,
	pub value: CommandOptionChoiceValue,
}

enum CommandOptionChoiceValue {
	String(String),
	Integer(i64),
	Number(f64),
}

This way if/when new choice variants are added (or possibly new fields) we don't need to update name() and name_localization() getters.

[vilgotf]

I think that's an even better change!

[vilgotf]

A very nice improvement!
One thing I noticed in the builder API is that we silently discard localizations if the choice is not found. It might be better to panic to alert users of their buggy code. Something like:

    /// ...
    /// # Panics
    ///
    /// Panics if the choice is not set.
    /// ...
    #[track_caller]
    pub fn choice_localizations<K: Into<String>, V: Into<String>>(
        mut self,
        choice_name: &str,
        name_localizations: impl IntoIterator<Item = (K, V)>,
    ) -> Self {
        let choice = self
            .0
            .choices
            .as_mut()
            .expect("choices are set")
            .iter_mut()
            .find(
                |choice| matches!(choice, CommandOptionChoice{ name, value: CommandOptionChoiceValue::Integer(_), ..}  if name == choice_name),
            ).expect("choice is set");
        // ...

[vilgotf]

LGTM, I pushed some small changes that I did not felt warranted a second review round