Legal API - updates to support currency_additional

**Tasks**
- [ ] Update to return currency_additional in json responses for share classes
- [ ] Update validation as req'd. May need to ensure `currency_additional` is not provided for new share classes or updates to share classes

Start by locating the Legal API share-class JSON response and validation entry points. Trace how existing and new share classes handle currency_additional, then verify responses and validation for both share-class creation and updates, including rejection when the field is not allowed.

Written by the indexing model from the issue text.