feat: ORDER BY CASE

[levkk]

fix #1736, specifically the ORDER BY CASE without aggregates use case.

[codecov]

Codecov Report

❌ Patch coverage is 67.88321% with 88 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...end/client/query_engine/test/rewrite_projection.rs	27.27%	32 Missing ⚠️
pgdog/src/frontend/router/parser/order_by.rs	60.86%	18 Missing ⚠️
...d/router/parser/rewrite/statement/order_by/case.rs	80.00%	11 Missing ⚠️
...nd/router/parser/rewrite/statement/order_by/mod.rs	73.17%	11 Missing ⚠️
...outer/parser/rewrite/statement/aggregate/engine.rs	52.38%	10 Missing ⚠️
.../router/parser/rewrite/statement/projection/mod.rs	95.74%	2 Missing ⚠️
pgdog/src/frontend/router/parser/route.rs	0.00%	2 Missing ⚠️
...og/src/backend/pool/connection/multi_shard/test.rs	0.00%	1 Missing ⚠️
...og/src/frontend/client/query_engine/route_query.rs	50.00%	1 Missing ⚠️

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