First aggregate function support

It would be nice to have FIRST() aggregate function that returns the first value in a bucket (similar to [LASTWITHTIME](https://github.com/apache/pinot/commit/da3bce50eb79656d967da7636a741653735abd63) function).

cc @Jackie-Jiang @weixiangsun 