feat: add table update to write builder

[QuakeWang]

Purpose

Linked issue: Part of #232

DataEvolutionWriter was exposed as a lower-level writer and DataFusion DML created it directly, while WriteBuilder only covered write and commit construction. This made the table-level write API incomplete and kept UPDATE / MERGE INTO data-evolution paths outside the builder flow.

This PR introduces TableUpdate as the table-level update API and wires data-evolution UPDATE / MERGE INTO through WriteBuilder::new_update(...).

Brief change log

• Add public TableUpdate as a thin wrapper around DataEvolutionWriter.
• Add WriteBuilder::new_update(update_columns).
• Export TableUpdate from paimon::table and crate root.
• Update DataFusion data-evolution UPDATE / MERGE INTO to create update writers through WriteBuilder.
• Reuse one WriteBuilder for MERGE INTO update writer, insert writer, and commit.

Tests

• cargo fmt --all -- --check
• cargo clippy -p paimon -p paimon-datafusion --all-targets --features fulltext,vortex -- -D warnings
• cargo test -p paimon table::write_builder
• cargo test -p paimon table::data_evolution_writer
• cargo test -p paimon-datafusion --test update_tests
• cargo test -p paimon-datafusion --test merge_into_tests

API and Format

Adds public TableUpdate and WriteBuilder::new_update(...).

Documentation