Implement unparse OuterReferenceColumn to String

[goldmedal]

Which issue does this PR close?

Closes #10523

Rationale for this change

IMO, OuterReferenceColumn is a column can be resolved to an outside field of the current plan. Typically, it's produced from identifier or compound_identifier.

datafusion/datafusion/sql/src/expr/identifier.rs

Lines 64 to 69 in 7535d93

	Ok((qualifier, field)) => {
	// found an exact match on a qualified name in the outer plan schema, so this is an outer reference column
	Ok(Expr::OuterReferenceColumn(
	field.data_type().clone(),
	Column::from((qualifier, field)),
	))

I think we can just convert it to a column sql node. Just like other identifier. Because the concept of outer reference is a logical plan concept, we can ignore the type of a OuterReferenceColumn which produced by a field resolving.

What changes are included in this PR?

Convert OuterReferenceColumn to a Column SQL node.

Are these changes tested?

Yes

Are there any user-facing changes?

No

[alamb]

LGTM -- thanks @goldmedal

This PR needs to be merged up from main to resolve a conflict if/when you have time

[alamb]

Thanks again @goldmedal

[goldmedal]

Thanks @alamb !