feat: introduce small object disk cache

[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

Changes:

• Introduce small object disk cache.
• Introduce large/small/mixed engine mode.
• Refine hybrid cache builder.
• Add large/small/mixed engine test on CI.

TODO:

• Small object disk cache destroy. (Maybe next time. The PR is large enough.)

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 84.81873% with 201 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
foyer-storage/src/small/generic.rs	63.24%	43 Missing ⚠️
foyer-storage/src/small/batch.rs	74.66%	38 Missing ⚠️
foyer-storage/src/store.rs	87.72%	27 Missing ⚠️
foyer-storage/src/small/set.rs	93.48%	23 Missing ⚠️
foyer-storage/src/small/set_manager.rs	84.21%	21 Missing ⚠️
foyer-storage/src/engine.rs	44.44%	20 Missing ⚠️
foyer-storage/src/small/flusher.rs	84.31%	16 Missing ⚠️
foyer-bench/src/main.rs	83.33%	5 Missing ⚠️
foyer-common/src/runtime.rs	0.00%	3 Missing ⚠️
foyer-storage/src/device/direct_file.rs	0.00%	1 Missing ⚠️
... and 4 more

Files with missing lines	Coverage Δ
foyer-storage/src/io_buffer_pool.rs	85.71% <ø> (ø)
foyer-storage/src/large/flusher.rs	85.14% <100.00%> (ø)
foyer-storage/src/large/reclaimer.rs	85.48% <100.00%> (ø)
foyer-storage/src/picker/mod.rs	100.00% <100.00%> (ø)
foyer-storage/src/picker/utils.rs	82.09% <100.00%> (ø)
foyer-storage/src/serde.rs	86.04% <100.00%> (+1.83%)	⬆️
foyer-storage/src/small/serde.rs	100.00% <100.00%> (ø)
foyer/src/hybrid/builder.rs	95.15% <100.00%> (+2.32%)	⬆️
foyer/src/hybrid/cache.rs	82.76% <100.00%> (+0.10%)	⬆️
foyer-storage/src/device/direct_file.rs	84.91% <0.00%> (ø)
... and 13 more

... and 2 files with indirect coverage changes