Make AggregateFunction take a single argument

[Dandandan]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
This makes the representation of aggregate functions more correct, makes code dealing with aggregate functions more simple.

Describe the solution you'd like
Instead of Vec<Expr> use Box<Expr>

Describe alternatives you've considered
Keeping it as is.

Additional context

[jorgecarleitao]

fyi, the goal of being Vec was to allow multi-argument aggregates. An example is the pearson correlation between two columns (which can currently be implemented via a UDAF)

[Dandandan]

@jorgecarleitao Ah, yeah I think that makes sense. I'll just take care of it.

I think there is still some code (e.g. in Ballista protobuf I believe) that assumes only 1 argument, so when we are going to support that, it should be adapted.