4656: Adding a new `searchableAttribute` no longer re-index all the attributes r=ManyTheFish a=Kerollmops
Fixes#4492.
## To Do
- [x] Do not call the `InnerSettingsDiff::only_additional_fields` function too many times
- [ ] Add tests
Co-authored-by: Clément Renault <clement@meilisearch.com>
Co-authored-by: ManyTheFish <many@meilisearch.com>