refactor: separate store and storage abstraction

[MrCroxx]

What's changed and what's your intention?

Please explain IN DETAIL what the changes are in this PR and why they are needed. :D

The PR separates the Store and the Storage abstraction.

It is the first step to support both large object store and the small object store.

Checklist

• I have written the necessary rustdoc comments
• I have added the necessary unit tests and integration tests
• I have passed make all (or make fast instead if the old tests are not modified) in my local environment.

Related issues or PRs (optional)

#596

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 85.78%. Comparing base (fa517db) to head (4955d36).

Files	Patch %	Lines
foyer-storage/src/picker/utils.rs	55.55%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #602      +/-   ##
==========================================
- Coverage   85.85%   85.78%   -0.07%     
==========================================
  Files          59       59              
  Lines        7577     7583       +6     
==========================================
  Hits         6505     6505              
- Misses       1072     1078       +6     

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