[core] Parallelize manifest reads in snapshot expiration

[leaves12138]

Purpose

Snapshot expiration currently reads manifest data serially in several cleanup paths. This can make expiration slow when IO and CPU resources are sufficient, especially for tables with many snapshots, manifest lists, and index manifests.

Changes

• Parallelize reading data manifest entries during data file cleanup while preserving the original manifest order for merge/delete decisions.
• Parallelize reading manifest lists and index manifests across snapshots during metadata cleanup, then apply deletion decisions sequentially against the shared skipping set.
• Parallelize building the retained manifest/index skipping set across retained/tagged snapshots.
• Keep existing best-effort behavior for unavailable manifest lists and missing index manifests.

Tests

• mvn -N -Pfast-build -DskipTests install
• mvn -pl paimon-api,paimon-test-utils,paimon-common,paimon-codegen,paimon-codegen-loader,paimon-arrow,paimon-format -Pfast-build -DskipTests install
• mvn -pl paimon-core -Pfast-build -DskipTests compile
• mvn -pl paimon-core -Pfast-build -Dtest=FileDeletionTest,ExpireSnapshotsTest,IndexFileExpireTableTest test
• mvn -pl paimon-core -DskipTests validate