Add a list of models to the generated code

[bikeshedder]

As of now everything is fully typed and there is no proper "list of models" available.

---

Originally written by @jacobsvante:

(...) I'd like to request is adding an enum containing all the model names, and a way to go from those to the actual models. I had the need for this when creating a CLI for quickly firing off requests. As you can see in my example below it's quite cumbersome as of today:

#[derive(Debug, Clone, Copy, ValueEnum)]
pub(crate) enum SunspecModel {
    M1,
    M2,
    M3,
}

impl SunspecModel {
    async fn read(
        &self,
        ctx: &mut Context,
        models: &mut Models,
    ) -> anyhow::Result<Box<dyn std::fmt::Debug>> {
        let config = sunspec::Config::default();
        match self {
            SunspecModel::M1 => Ok(Box::new(read_model(ctx, &models.m1, &config).await?)),
            SunspecModel::M2 => Ok(Box::new(read_model(ctx, &models.m2, &config).await?)),
            SunspecModel::M3 => Ok(Box::new(read_model(ctx, &models.m3, &config).await?)),
        }
    }
}

Originally posted by @jacobsvante in #9 (comment)