refactor(agg): separate semantics of AggKind and AggType

[stdrc]

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What's changed and what's your intention?

Just a preparation for fixing UDAF as window function. #18436

The rationale is that I'll introduce a new proto message called AggType which contains agg_kind and other metadata like UDF meta or wrapped scalar expression node.

So after this, the word kind will be used for detailed, simple enum types, type will be used for high-level classes that can be either simple or compound.

Checklist

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[stdrc]

• refactor(agg): separate semantics of AggKind and AggType #18640 👈
• main

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[BugenZhao]

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