feat: add support for having clause

[murex971]

Summary

• Add support for HAVING in cross-shard GROUP BY queries.
• Introduce HavingRewritePlan to inject hidden HAVING dependencies (when not in SELECT).
• Update multi-shard processing order to run: aggregate -> having -> sort/distinct/limit, then drop hidden columns before returning rows/row description to clients.

Testing

Added unit and integrated tests

Related Issues

closes #695

[codecov]

Codecov Report

❌ Patch coverage is 67.06408% with 221 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/router/parser/having.rs	59.30%	105 Missing ⚠️
pgdog/src/backend/pool/connection/buffer.rs	63.39%	56 Missing ⚠️
...frontend/router/parser/rewrite/statement/having.rs	76.88%	52 Missing ⚠️
...dog/src/backend/pool/connection/multi_shard/mod.rs	55.55%	8 Missing ⚠️

📢 Thoughts on this report? Let us know!

[levkk]

This will require a thorough review. Will try to circle back next week due to #1114 (comment)