Add Sendable Conformances (Depends on #35)

[roanutil]

I think we are using subscription types in a safe way but they are not easily made Sendable. Let me know if you see anything that could be problematic with @unchecked Sendable.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 49.27536% with 35 lines in your changes missing coverage. Please review.

Project coverage is 82.33%. Comparing base (dc094d0) to head (f479476).
Report is 17 commits behind head on 3.0-preview.

Files with missing lines	Patch %	Lines
Sources/CoreDataRepository/CoreDataError.swift	0.00%	33 Missing ⚠️
...es/Internal/ModelsWithIntId/ManagedModel_Int.swift	0.00%	1 Missing ⚠️
...rces/Internal/NSManagedObjectModel+Constants.swift	50.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           3.0-preview      #36      +/-   ##
===============================================
- Coverage        84.26%   82.33%   -1.93%     
===============================================
  Files               35       65      +30     
  Lines             2936     7315    +4379     
===============================================
+ Hits              2474     6023    +3549     
- Misses             462     1292     +830     

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