Refactor aggregation function structs

[systay]

Our parser outputs all aggregations functions using the general sqlparser.FuncExpr. This is a little clunky and requires us to do additional checks before we are sure that the aggregation we have is valid and what it is aggregating.

I suggest we instead introduce one struct type per aggregation, and have them all implement some interface. This would help clean up the code.

Example:

type (
	AggrFunc interface {
		iAggrFunc()
	}

	CountStar struct {}

	Count struct {
		Arg Expr
	}
        // and the rest
)

Tasks:

• Add AST structs
• Update the parser to use these structs
• Find and replace uses of the old representation and use the new structs instead

[skant7]

Hi @systay is there any chance to include this issue as one of the projects for summer 2022 lfx mentorship ?

[AllMight2099]

Hey @rsajwani, I'd like to work on this issue if that's alright with you

[GuptaManan100]

I think @rsajwani already started working on it and the associated PR is #10347. @rsajwani can update on the progress, and whether we can close this issue or if some part of the work is left.

[AllMight2099]

Oh okay. @GuptaManan100 Are there any good first issues that you'd recommend?

[GuptaManan100]

@AllMight2099 We have a Good First Issue label in Vitess. You can try filtering out issues with that, or you could pick up something for the list ☝️