[lake/tiering] Support reporting watermark to Paimon snapshot

[Shawn-Hx]

Purpose

Linked issue: close #3419

This PR supports reporting watermarks to Paimon snapshots during lake tiering.

Brief change log

• Introduced watermark extraction for Fluss rows in the lake tiering pipeline.
• Added watermark propagation through lake write results and tiering commit.
• Aggregated watermarks across tiered buckets before committing to the lake.
• Reported the aggregated watermark to Paimon snapshots when committing.
• Updated serializers and tests to support nullable and negative watermark values.
• Added coverage for watermark extraction, commit aggregation, and Paimon snapshot watermark reporting.

Tests

Add tests in:

• PaimonWriteResultSerializerTest
• PaimonTieringTest
• TieringCommitOperatorTest
• SimpleWatermarkExtractorTest

API and Format

This change extends the lake tiering API to carry watermark information through write results and committers.

Documentation

No user-facing documentation needs to be added.