perf: use size hint on serde

[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

As title. To reduce buffer growing.

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)

#694

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
foyer-storage/src/serde.rs	94.73%	1 Missing ⚠️

Files with missing lines	Coverage Δ
foyer-storage/src/store.rs	74.72% <100.00%> (ø)
foyer-storage/src/serde.rs	96.42% <94.73%> (-3.58%)	⬇️

[MrCroxx]

LGTM