[PYG-407] 🐜As write fails for CDF external references. (#499)

# Description

The current as write logic fails to account for
TimeSeries/FileMetadata/Sequence references in the view.

## Bump

- [x] Patch
- [ ] Minor
- [ ] Skip

## Changelog
### Fixed

- In the generated SDK, the `.as_write(...)` field no longer fails for
data classes that uses CDF external reference fields.
