Add `NotebookIndex` to the cache (#6863)
## Summary
This PR updates the `FileCache` to include an optional `NotebookIndex`
to support caching for Jupyter Notebooks.
We only require the index to compute the diagnostics and thus we don't
really need to store the entire `Notebook` on the `Diagnostics` struct.
This means we only need the index to be stored in the cache to
reconstruct the `Diagnostics`.
## Test Plan
Update an existing test case to run over the fixtures under
`ruff_notebook` crate where there are multiple Jupyter Notebook.
Locally, the following commands were run in order:
1. Remove the cache: `rm -rf .ruff_cache`
2. Run without cache: `cargo run --bin ruff -- check --isolated
crates/ruff_notebook/resources/test/fixtures/jupyter/unused_variable.ipynb
--no-cache`
3. Run with cache: `cargo run --bin ruff -- check --isolated
crates/ruff_notebook/resources/test/fixtures/jupyter/unused_variable.ipynb`
4. Check whether the `.ruff_cache` directory was created or not
5. Run with cache again and verify: `cargo run --bin ruff -- check
--isolated
crates/ruff_notebook/resources/test/fixtures/jupyter/unused_variable.ipynb`
## Benchmarks
https://github.com/astral-sh/ruff/pull/6863#issuecomment-1715675186
fixes: #6671
