Enable scalar function aggregation in PPL

[dai-chen]

Is your feature request related to a problem? Please describe.

opendistro-for-elasticsearch/sql#1079

Describe the solution you'd like
Actually the core engine already has this capability. What is needed is just to turn on this in ANTLR grammar in PPL.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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[dai-chen]

I checked PPL grammar that most commands (sort, eval etc) only accept field rather than expression. Need to double check if this is by design.