feat: support indexing AbstractNonlinearSolveCache

[AayushSabharwal]

Close https://github.com/SciML/SciMLBase.jl/issues/662

@TorkelE

Checklist

• [ ] Appropriate tests were added
• [ ] Any code changes were done in a way that does not break public API
• [ ] All documentation related to code changes were updated
• [ ] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [ ] Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.14%. Comparing base (e3238a7) to head (b70fbe1).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #406 +/- ## ========================================== - Coverage 86.59% 80.14% -6.46% ========================================== Files 46 46 Lines 2827 2840 +13 ========================================== - Hits 2448 2276 -172 - Misses 379 564 +185 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.