[Discuss] Different implementation style between Expr, LogicalPlan and ExecutionPlan

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In current DataFusion code base, `Expr`, `LogicalPlan` and `ExecutionPlan` are represented in different ways. Actually they are all tree based structures. `Expr` and `LogicalPlan` are Enums, `ExecutionPlan` is a Trait.
The LogicalPlan enum wrapped the different logical operator structs like Projection and Filter, etc.

````
pub enum LogicalPlan {
Projection(Projection),
Filter(Filter),
............
}
````

But the Expr enum doesn't wrap the expression structs and define the different expressions directly in the enum.

````
pub enum Expr {
Alias(Box, String),
Column(Column),
............
AggregateFunction {
fun: aggregate_function::AggregateFunction,
args: Vec,
distinct: bool,
},
WindowFunction {
fun: window_function::WindowFunction,
args: Vec,
partition_by: Vec,
order_by: Vec,
window_frame: Option,
},
............
}
````

I think we should unify the coding style, at least the Expr and LogicalPlan representations should follow the same style. And for physical ExecutionPlan, it is Trait/Trait Objects, I would prefer to use Enum also. Want to hear thoughts from other members.

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Start by comparing the Expr, LogicalPlan, and ExecutionPlan definitions described in the issue and review the surrounding tree representations. The issue is a design discussion rather than a bounded implementation task; done would require agreement on a unified representation and a defined migration scope.

Written by the indexing model from the issue text.