feat: add `ParameterIndexingProxy`

[AayushSabharwal]

Checklist

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

Additional context

Add any other context about the problem here.

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (f407c15) 24.85% compared to head (9189b4d) 24.30%.

Files	Patch %	Lines
src/parameter_indexing_proxy.jl	0.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #26 +/- ## ========================================== - Coverage 24.85% 24.30% -0.55% ========================================== Files 6 7 +1 Lines 177 181 +4 ========================================== Hits 44 44 - Misses 133 137 +4 ```

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