[Postgres Support 5/5] Python ch_sink_tools Package

[minguyen9988]

Summary

Restructures the Python tooling into a proper ch_sink_tools package with ANTLR-generated PostgreSQL parsers, database comparison utilities, schema dumping, and data loading tools.

Part 5 of 5 PRs split from #1245 for reviewability.

Changes

• Package restructure: Reorganized from flat db/, db_compare/, db_dump/, db_load/ directories into proper ch_sink_tools Python package with submodules
• ANTLR PostgreSQL grammar: Added PostgreSQL SQL grammar files and generated Python parser/lexer
• PostgreSQL support: db/postgres.py for PostgreSQL connections, postgres_type_mapper.py for type mapping, postgres_dumper.py for schema dumping
• Build tooling: pyproject.toml for modern Python packaging, build_wheel.sh for wheel builds
• Config system: column_type_overrides.py and override_reconciler.py for column type override configuration
• Database comparison: Updated db_compare module for PostgreSQL↔ClickHouse comparison
• Tests: test_naming.py for naming utility tests

Files (66 files, +106,547/−110)

• 37 new files (ANTLR grammars, generated parsers, new modules)
• 21 renamed files (package restructure)
• 4 modified files (README, build scripts, requirements)
• 4 deleted files (old init.py files)

Review Notes

• Independent of Java PRs (1-4) — can be reviewed and merged separately
• Large line count is primarily from ANTLR-generated parser files
• The package restructure (renames) makes this a clean break from the old flat structure