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

crates/ruff_cli/src/commands/check.rs

@@ -11,6 +11,7 @@ use itertools::Itertools;
 use log::{debug, error, warn};
 #[cfg(not(target_family = "wasm"))]
 use rayon::prelude::*;
+use rustc_hash::FxHashMap;
 
 use ruff::message::Message;
 use ruff::registry::Rule;
@@ -156,6 +157,7 @@ pub(crate) fn check(
                                 TextSize::default(),
                             )],
                             ImportMap::default(),
+                            FxHashMap::default(),
                         )
                     } else {
                         warn!(