Support multi-arg AggregateFunction types

[gnzjgo]

Summary

• Extend t.aggregateFunction to accept one or more state type validators
• Emit AggregateFunction(func, T1, T2, ...) for multi-argument aggregate states like argMax
• Update ClickHouse type codegen to round-trip multi-type AggregateFunction(...) definitions

Closes #157.

Verification

• pnpm vitest run src/schema/types.test.ts src/codegen/type-mapper.test.ts
• pnpm typecheck
• pnpm test:run