[rust] Cache decoded BLOB indexes across readers

[XiaoHongbo-Hope]

Purpose

Opening the same immutable .blob file with a new reader currently reloads its footer and index, adding two object-store range reads each time.

This caches the decoded index across readers, following the bounded process-local cache used by PyPaimon in apache/paimon#9547.

Changes

• Cache up to 16 decoded BLOB indexes in a process-local LRU.
• Share immutable indexes with Arc; readers and BLOB payloads are not cached.
• Keep cache locks out of asynchronous I/O.
• Verify that reopening the same file performs no footer or index reads.

Trade-offs

• Cold reads are unchanged.
• Cache hits avoid two range requests per BLOB file.
• Up to 16 decoded indexes remain in memory.
• This relies on Paimon data file paths being immutable; there is no API or format change.

Tests

• cargo test -p paimon --lib blob (139 passed)
• cargo clippy -p paimon --lib -- -D warnings
• cargo fmt --all --check
• git diff --check