feat: add write builder configuration

[QuakeWang]

Purpose

Part of #232.

The current write path configures some writer behavior after creating TableWrite, while TableCommit is created independently from the same WriteBuilder. This makes builder-level write configuration incomplete and prevents writers and committers from sharing an explicit commit user.

Brief change log

• Add WriteBuilder::with_commit_user so writers and committers created from the same builder share the same commit user.
• Validate custom commit users as safe file name segments, because the value is persisted in snapshot metadata, used for duplicate commit detection, and embedded in postpone-bucket data file name prefixes.
• Add WriteBuilder::with_overwrite and route DataFusion overwrite writers through the builder instead of mutating TableWrite after creation.
• Add focused tests for commit user sharing, invalid commit user rejection, and overwrite-aware writer creation.

Tests

• cargo test -p paimon table::write_builder
• cargo test -p paimon table::table_write
• cargo test -p paimon table::table_commit
• cargo test -p paimon-datafusion table::tests::test_insert_overwrite

API and Format

Adds public WriteBuilder APIs: commit_user, with_commit_user, and with_overwrite.

No storage format change.

Documentation

Public API doc comments document the commit_user semantics and validation requirements.