Continue to reduce Expr struct size

### Is your feature request related to a problem or challenge?

Currently, we already done some work for reducing the Expr size in:

https://github.com/apache/datafusion/pull/16207

We can continue to reduce the Expr struct size

### Describe the solution you'd like

change Expr Alias and OuterReferenceColumn to Box type for reducing expr struct size

### Describe alternatives you've considered

change Expr Alias and OuterReferenceColumn to Box type for reducing expr struct size

### Additional context

_No response_

Review PR #16207 first, then locate the Rust definition and uses of Expr, Alias, and OuterReferenceColumn to trace how these variants are constructed and matched. Run the relevant DataFusion tests after the change; done means the Expr representation is smaller without behavioral regressions.

Written by the indexing model from the issue text.