feat: add ProblemState

[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]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 0.00%. Comparing base (494548d) to head (c1b60a6).

Files	Patch %	Lines
src/problem_state.jl	0.00%	6 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master     #64   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files           7       8    +1     
  Lines         366     372    +6     
======================================
- Misses        366     372    +6     

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[AayushSabharwal]

Docs won't build till MTK is tagged