feat: combined disk cache framework (p2)

[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

This PR is the second part to implement the combined (large + small) disk cache.

Changes:

• Introduced Device enum, renamed the original Device to Dev.
• Refined the combined disk cache framework.
• Removed generic type Dev from most of the types.

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 50.57143% with 173 lines in your changes missing coverage. Please review.

Project coverage is 82.79%. Comparing base (37fd364) to head (4015c70).

Files	Patch %	Lines
foyer-storage/src/store.rs	39.85%	83 Missing ⚠️
foyer-storage/src/engine.rs	23.46%	75 Missing ⚠️
foyer-storage/src/device/mod.rs	65.85%	14 Missing ⚠️
foyer-storage/src/large/generic.rs	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #605      +/-   ##
==========================================
- Coverage   83.66%   82.79%   -0.87%     
==========================================
  Files          62       62              
  Lines        7843     7976     +133     
==========================================
+ Hits         6562     6604      +42     
- Misses       1281     1372      +91     

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