[dotnet] Add mixin for easier state save/load apis

[jackgerrits]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.97%. Comparing base (48242f5) to head (f539b19).

Files with missing lines	Patch %	Lines
...tnet/src/Microsoft.AutoGen/Contracts/ISaveState.cs	0.00%	2 Missing ⚠️
...src/Microsoft.AutoGen/Contracts/ISaveStateMixin.cs	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5438      +/-   ##
==========================================
+ Coverage   71.91%   71.97%   +0.05%     
==========================================
  Files         236      238       +2     
  Lines       11922    11927       +5     
  Branches      238      239       +1     
==========================================
+ Hits         8574     8584      +10     
+ Misses       3163     3157       -6     
- Partials      185      186       +1     

Flag	Coverage Δ
unittests	71.97% <57.14%> (+0.05%)	⬆️

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