Reduce Field Copy operations before releasing 48.0.0 #16121

Reduce the field copy operations in response to this PR comment:

The only thing I am worried about with this change is the number of Fields that are passed around -- each Field contains an owned String so each time we copy a Field it will force a copy of the String and will potentially slow down planning, especially for large and complex queries.

Do you think it is feasible to update the scalar, aggregate, and window function APIs to use FieldRef instead of Field? That way we can avoid most string copies.

Originally posted by @alamb in #15911 (review)