feat: write temp buffer before engine enqueue

[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

Serialize cache entry to a temporary buffer before enqueue to engine.

This method makes the serialized entry size known before submitting it to the engine, making it possible to decide which engine to use with the serialized entry size.

But this PR may increase some memory usage when the buffer is waiting to be written to the main buffer. Maybe use parallel buffer writing.

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 86.74699% with 22 lines in your changes missing coverage. Please review.

Project coverage is 82.70%. Comparing base (8f7baf6) to head (d45d692).

Files	Patch %	Lines
foyer-storage/src/storage/either.rs	0.00%	9 Missing ⚠️
foyer-storage/src/engine.rs	58.33%	5 Missing ⚠️
foyer-storage/src/small/generic.rs	0.00%	4 Missing ⚠️
foyer-storage/src/store.rs	88.88%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #608      +/-   ##
==========================================
- Coverage   82.76%   82.70%   -0.07%     
==========================================
  Files          62       62              
  Lines        7986     8016      +30     
==========================================
+ Hits         6610     6630      +20     
- Misses       1376     1386      +10     

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