Fix `AssetDetails` endpoint to match GraphQL standards (#3070)

## Linked Issues/PRs
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https://github.com/FuelLabs/fuel-core/issues/3069

## Description
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## Checklist
- [x] Breaking changes are clearly marked as such in the PR description
and changelog
- [x] New behavior is reflected in tests

### Before requesting review
- [x] I have reviewed the code myself