Logical expression ScalarVariable cannot represent an extension type or metadata

[paleolimbot]

Is your feature request related to a problem or challenge?

The ScalarVariable is represented in terms of DataType, and thus plans can't

datafusion/datafusion/expr/src/expr.rs

Lines 313 to 314 in 6ecf76c

	/// A named reference to a variable in a registry.
	ScalarVariable(DataType, Vec<String>),

While we are here, it would probably be a good idea to allow the nullability to be represented here as well as most other logical expression members can store this, too.

Describe the solution you'd like

Perhaps the DataType could be updated to a FieldRef or FieldMetdata could be added

Describe alternatives you've considered

No response

Additional context

• Issue for updating Cast: LogicalPlan Casts can't express a cast to an extension type #18060
• Issue for updating Placeholder: Logical plan placeholders can't contain metadata #17862