4769: Federated search r=ManyTheFish a=dureuill
# Pull Request
## Related issue
Fixes#4747
[Usage](https://meilisearch.notion.site/v1-10-federated-search-698dfe36ab6b4668b044f735fb40f0b2)
## What does this PR do?
- multi-search now allows a top-level federation object. When not `null`, the results of multi-search are modified to be a single list of results rather than a list of a list of results
- changed lifetimes around tokenizer et al. to be able to make hits one by one rather than using a vector
- adds `roaring` to Meilisearch itself. As the federated search happens at the Meilisearch level (reuses the search functions declared at the Meilisearch level + merge happens after the hits were created), `RoaringBitmap`s are needed to track the candidates: hits that were seen, all candidates.
- Refactor `make_hits` to allow for an individual, optimized `make_hit`
- Score details comparison no longer fail when sorting on different field names or target point (for geo)
Co-authored-by: Louis Dureuil <louis@meilisearch.com>
4787: Add index exists function in index_scheduler which stops opening indexes to only check if they exist. r=Kerollmops a=Karribalu
# Pull Request
## Related issue
Fixes#4784
## What does this PR do?
- Added index_exists function in the index_scheduler.
- Resolved opening indexes to only check if they exist.
- Made changes to existing tests to test this function.
## PR checklist
Please check if your PR fulfills the following requirements:
- [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
- [x] Have you read the contributing guidelines?
- [x] Have you made sure that the title is accurate and descriptive of the changes?
Thank you so much for contributing to Meilisearch!
Co-authored-by: karribalu <karri.balu123456@gmail.com>
