feat: flush hook extension point

[sunng87]

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What's changed and what's your intention?

This patch adds extension points and plugin resgistration for flush/compaction.

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.

[gemini-code-assist]

Code Review

This pull request introduces a FlushHook extension point for the mito2 engine, allowing plugins to receive notifications when SST files are written or the manifest is updated during flush and compaction operations. The implementation includes a new PkCollectingIter to gather unique primary keys during these processes. Feedback from the review highlights a critical bug in the primary key column indexing logic and a serialization issue where hook-related data is skipped in MergeOutput, which would break remote compaction. Additionally, there are suggestions to optimize performance by using references instead of cloning FileMeta and a warning regarding potential memory exhaustion when collecting a large volume of primary keys.