Avoid a full index deletion in Transform when changing settings

[ManyTheFish]

Related product team resources: PRD (internal only)

Summary

This issue is a subset of the work implementing the settings diff-indexing enhancement.

The current approach of Transform::prepare_for_documents_reindexing is to dump all the documents in the index into a Grenad::File and delete the whole index to reindex all the documents, taking into account the Settings changes.

This function should instead create document deltas without clearing the index.

TODO

• Ensure Meilisearch has tests triggering the Transform::prepare_for_documents_reindexing methods
• Make both into_del_add_obkv calls use DelAddOperation::DeletionAndAddition
• Remove ClearDocuments calls
  

[ManyTheFish]

fixed by #4504