Postgresql 19 support and some bug fixes

What's Changed

• Enforce thresholds in explicit representation handling by @colm-mchugh in #174
• Support PostgreSQL 19 by @CyberDem0n in #173
• Bump extension version to 2.20 by @ihalatci in #175

New Contributors

• @colm-mchugh made their first contribution in #174
• @CyberDem0n made their first contribution in #173

Full Changelog: v2.19...v2.20