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>
