feat: tracks index files in another cache and preloads them

[evenyag]

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What's changed and what's your intention?

This PR implements an dedicated file cache for puffin files. It adds the following two options:

• preload_index_cache: Preload index (puffin) files into cache on region open. This is enabled by default.
• index_cache_percent: Percentage of write cache capacity allocated for index (puffin) files.

The FileCache adds another index to track puffin files under the write cache. When the engine opens a region, it creates a RegionLoadCacheTask to load the puffin files. The WriteCache runs the RegionLoadCacheTask after it finish recovery.

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.