Rust: Allow specifying of Custom use in header and #[derives] on Structs

[AdamLeyshon]

This would enable people to use the generated Structs with other libraries such as Diesel that require us to use #[derive] macros on Structs that have complicated trait implementations and others such as Default

Example:

#[derive(FromSqlRow, AsExpression, Serialize, Deserialize, Debug, Default)]
#[serde(deny_unknown_fields)]
pub struct ApiConfigData {
    #[serde(rename = "inventory")]
    inventory: Inventory,

    #[serde(rename = "maintenance")]
    maintenance: Maintenance,
}