Unparsing optimized (> 2 inputs) unions

[MohamedAbdeen21]

Which issue does this PR close?

Closes #13621.

Rationale for this change

Unparsing unions with more than 2 inputs (produced by the logical optimizer)

What changes are included in this PR?

Tests and required changes to unparser

Are these changes tested?

Yes

Are there any user-facing changes?

Should be able to unparse most unions

[MohamedAbdeen21]

Looks like there's a circular dep between optimizer and SQL packages. The easiest solution is moving the test somewhere else, not sure where though

[goldmedal]

Thanks for working on this, @MohamedAbdeen21
Instead of applying optimization rules when roundtrip tests, I prefer to construct the expected plan manually. Just like other test cases like

datafusion/datafusion/sql/tests/cases/plan_to_sql.rs

Line 1221 in 482b489

fn test_join_with_table_scan_filters() -> Result<()> {

If we add the optimization to the testing, the optimizer's behavior will be limited by the unparser but we don't expect it. (see the previous discussion #13267 (comment))

[MohamedAbdeen21]

Thanks for the suggestion @goldmedal, updated accordingly

[goldmedal]

Thanks @MohamedAbdeen21. Overall looks good to me. Just some minor comments.
cc @phillipleblanc

[phillipleblanc]

Thanks for fixing this @MohamedAbdeen21!

[MohamedAbdeen21]

@goldmedal reverted let Some(..) .. else so we can correctly error on union w/ 1 input.

[goldmedal]

Thanks @MohamedAbdeen21 and @phillipleblanc for the review