feat: add reason for connection closing

[levkk]

Helps with debugging settings.

[codecov]

Codecov Report

❌ Patch coverage is 65.30612% with 17 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/backend/disconnect_reason.rs	50.00%	8 Missing ⚠️
pgdog/src/backend/pub_sub/listener.rs	0.00%	4 Missing ⚠️
pgdog/src/backend/pool/inner.rs	88.23%	2 Missing ⚠️
pgdog/src/backend/server.rs	80.00%	2 Missing ⚠️
pgdog/src/backend/pool/pool_impl.rs	0.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!