⛓Use YAML classes in the validation of assets, events, filemetadata, dataset, and labels (#1648)

# Description

Please describe the change you have made.

## Changelog

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

## cdf

### Improved

- In the `cdf build` command, Toolkit now gives improved warning
messages if the syntax in the configuration file is incorrect for
`assets`, `events`, `filemetadata`, `dataset`, and `labels`.

## templates

No changes.
