minor: Refactor binary expr serde to reduce code duplication

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

We currently have 20 duplicate structs defined in protobuf for binary expressions, such as:

message Equal {
  Expr left = 1;
  Expr right = 2;
}

message NotEqual {
  Expr left = 1;
  Expr right = 2;
}

message EqualNullSafe {
  Expr left = 1;
  Expr right = 2;
}

As a result, we have a lot of duplicate code in QueryPlanSerde for serializing binary expressions that cannot easily be refactored because each code block references a different (but identical) generated class.

What changes are included in this PR?

• Use a single definition of BinaryExpr
• Refactor QueryPlanSerde to remove a lot of duplicate boilerplate code

How are these changes tested?

Existing tests

[andygrove]

I will follow up with similar PRs for UnaryExpr and MathExpr

[andygrove]

Thanks for the reviews @kazuyukitanimura @parthchandra @parthchandra