Metafield value support or documentation

I found it confusing to use the shopify_function crate for the metafield data types being passed between Shopify and the Function.

One issue was the inability to preview / see the generated types, which I see an existing issue for: https://github.com/Shopify/shopify-function-rust/issues/17

Here is an example of a function API that sends metafields: https://shopify.dev/docs/api/functions/reference/order-routing-location-rule/graphql/common-objects/metafield

The metafield value is passed from Shopify to the function as a string and I found the support for this in shopify_function not clear.

When not using this crate and generating types myself in api.rs, I ended up doing something like this to handle the metafield format:

pub struct Metafield {
    pub value: Option<String>,
}

pub struct Configuration {
  pub value: Option<ExampleGroups>,
}

impl Configuration {
  fn from_str(value: &str) -> Self {
    // ExampleGroups::from_str constructs the JSON from the metafield string value
    Configuration { value: Some(ExampleGroups::from_str(&value)) }
  }

  fn default() -> Self {
    Configuration {
      value: None,
    }
  }
}

pub fn parse_function_configuration_value(metafield: Option<Metafield>) -> Configuration {
  match metafield.and_then(|m| m.value) {
    Some(value) => Configuration::from_str(&value),
    None => Configuration::default(),
  }
}

Start by reviewing the shopify_function crate's metafield handling and the generated types concern in issue #17. Compare this with the referenced Shopify Function API example and the custom types shown in api.rs; done should be a clear documented path for handling metafield values, or a defined support change if documentation is insufficient.