feat: add partial-update merge engine groundwork

[QuakeWang]

Purpose

part of: #256

This PR just for partial-update merge engine without claiming end-to-end support yet, and focuses on parsing, validation, merge-core preparation, and fail-fast safety rails so Rust Paimon does not silently read or write partial-update tables with incorrect semantics.

Brief change log

• add MergeEngine::PartialUpdate parsing support
• introduce PartialUpdateConfig to centralize partial-update option validation
• reject unsupported advanced partial-update options such as sequence-group, aggregate-function, and partial-delete related options
• add fail-fast checks for PK partial-update reads and writes
• disable limit pushdown hints for PK partial-update tables
• generalize the sort-merge contract so merge functions can return materialized rows, preparing the merge core for future field-wise partial-update implementation
• add regression tests to keep existing first-row and current merge behavior stable

Tests

API and Format

Documentation