feat: add `default_values` to interface

[AayushSabharwal]

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

Attention: Patch coverage is 5.26316% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 34.03%. Comparing base (2a45e9c) to head (7713bc0).

Files	Patch %	Lines
src/symbol_cache.jl	6.89%	27 Missing :warning:
src/trait.jl	0.00%	8 Missing :warning:
src/interface.jl	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #47 +/- ## ========================================== - Coverage 38.17% 34.03% -4.14% ========================================== Files 7 7 Lines 296 332 +36 ========================================== Hits 113 113 - Misses 183 219 +36 ```

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[AayushSabharwal]

Does the spec says what should be used as keys or is it up to the package that implements this?

All SII can say is that the keys are symbolic variables. Whether that's Num, Symbol, BasicSymbolic or something else is up to the implementation, since SII does not know of these

[AayushSabharwal]

Right now it's somewhat arbitrary. I don't think we'll need it to be mutable in remake or anywhere else, but you mentioned that we should standardize it and mutability offers us more flexibility in the future.

[SebastianM-C]

Yeah, it's nicer if it's mutable, that makes it easier in certain situations.