ConvertTo or CreateForm with constructor, factory or builder?

Hi!

I’ve recently started developing with Goa, and I’m still learning about all its functionalities. I’m currently trying to convert a user-defined type into a struct in a library. For this specific example, I want to handle a currency code and amount, using the [currency](https://github.com/bojanz/currency) library for internal representation.

Here’s the code I’ve implemented so far. However, I noticed that the library internally uses private fields and only exposes constructors to create the `Amount` object:

```
var CurrencyType = Type("Currency", func() {
Description("Currency amount")
TypeName("CurrencyType")
ConvertTo(domain.Currency{})
Attribute("currencyCode", String, "Currency code in ISO 4217", func() {
Pattern(`^[A-Z]{3}$`)
Example("USD")
MaxLength(3)
MinLength(3)
})
Attribute("number", String, "Amount of currency", func() {
Pattern(`^\d+(\.\d{1,6})?$`)
Example("1234.56")
})
Required("currencyCode", "number")
})
```

The constructor provided by the library, which I find useful, is:

`
func NewAmount(n, currencyCode string) (Amount, error)
`

It’s straightforward to implement the conversion for the `CurrencyType` field in my own packages. However, when `CurrencyType` is part of a more complex user-defined type, I’m unsure how to specify the constructor instead of directly mapping fields during the conversion.

My question is: Is there a way to parameterize the constructor to map a specific field when using `ConvertTo`?