use admin api returnCalculate with multiple currencies

[ShaunStarttin]

The returnCalculate query specifies that the `returnShipping` input should be provided in the presentment currency. The current "return shipping rules" only allow for one value and the self-serve return page seems to automatically convert the value when it calculates return shipping. Is there a suggested way to convert an override value ourselves? I can't find a mutation, so assume we should handle it or allow a merchant to provide values.

Also, if you provide the wrong currency to the `returnShipping` input in this query or in returnRequest, they just return `null`. If you _dont_ provide a value, the default store value is applied.