Preserve field metadata across expressions in logical plans #6886

Is your feature request related to a problem or challenge?

I'm working on a project that requires using Arrow extension types. Because of the way extension types are encoded in arrow-rs (see apache/arrow-rs#4472) I have to keep track of type information at the field/schema level. This works at execution time, but I've run into an issue trying to extract metadata from a field during the planning phase as field metadata isn't being propagated.

This seems to originate in the ExprSchemable::to_field trait, where when a field is constructed from an expression it only copies the datatype and nullability from the input schema.

Describe the solution you'd like

A reasonable approach to me would be to add a metadata method to the ExprSchemable trait that would forward field metadata for type-preserving expressions (e.g. column, alias, etc.) and return empty metadata for others.
ExprSchemable::to_field could then be updated to pass the metadata to the DFField constructor.

Describe alternatives you've considered

The proposed solution would enable metadata propagation through expressions. I haven't gone through every logical plan node so it's possible I'm missing other places where metadata would be erased that would also need to be changed.

Additional context

No response