Migrate aggregation functions to function-v2

[RinChanNOWWW]

Summary

• [x] combinator distinct. #7615
• [x] combinator if. #7615
• [x] count. #7615
• [x] sum. #7615
• [x] avg. #7615
• [x] uniq. #7615
• [ ] min, max, any. #7787
• [ ] arg_min, arg_max.
• [ ] std, stddev, stddev_pop.
• [ ] covar_samp
• [ ] covar_pop.
• [ ] window_funnel.
• [ ] retention.

something may be helpful

• [ ] add try_downcast_builder for ValueType. #7838
• [ ] unit tests for group by. #7845
• [x] ~add Default for ValueType::Scalar or add default_scalar for ValueType.~

[RinChanNOWWW]

• add Default for ValueType::Scalar or add default_scalar for ValueType.
• add try_downcast_builder for ValueType.

Are these necessary? cc @sundy-li

[RinChanNOWWW]

The Default bound seems not necessary, and not suitable for AnyType. Let's not consider this.

[TCeason]

stddev and Window funnel assign to @TCeason