Unparse UNION plan with multiple inputs to SQL text

[goldmedal]

Is your feature request related to a problem or challenge?

Given a SQL like

WITH vals AS
  (
    SELECT 1 x, 'a' y UNION ALL
    SELECT 1 x, 'b' y UNION ALL
    SELECT 2 x, 'a' y UNION ALL
    SELECT 2 x, 'c' y
  )
SELECT x, y
FROM vals

The unoptimized plan is

 Projection: vals.x, vals.y
  SubqueryAlias: vals
    Union
      Union
        Union
          Projection: Int64(1) AS x, Utf8("a") AS y
            EmptyRelation
          Projection: Int64(1) AS x, Utf8("b") AS y
            EmptyRelation
        Projection: Int64(2) AS x, Utf8("a") AS y
          EmptyRelation
      Projection: Int64(2) AS x, Utf8("c") AS y
        EmptyRelation

It's fine for the unparser. However, after EliminateNestedUnion optimization rule, the nested union will be flatten to one union like

Projection: vals.x, vals.y
  SubqueryAlias: vals
    Union
      Projection: Int64(1) AS x, Utf8("a") AS y
        EmptyRelation
      Projection: Int64(1) AS x, Utf8("b") AS y
        EmptyRelation
      Projection: Int64(2) AS x, Utf8("a") AS y
        EmptyRelation
      Projection: Int64(2) AS x, Utf8("c") AS y
        EmptyRelation

If we tried to unparse this plan, we will get the error message:

Error: This feature is not implemented: UNION ALL expected 2 inputs, but found 4

Describe the solution you'd like

Currently, the unpaser only supports unparsing an union with 2 input

datafusion/datafusion/sql/src/unparser/plan.rs

Lines 630 to 635 in 172e557

	if union.inputs.len() != 2 {
	return not_impl_err!(
	"UNION ALL expected 2 inputs, but found {}",
	union.inputs.len()
	);
	}

We should support unparsing the union with more 2 inputs for the Unparser.

Describe alternatives you've considered

No response

Additional context

No response