Add schema-aware optimizer child rewrites

[sakshichitnis27]

Fixes #23666

Summary

• Add an optimizer-local helper for schema-aware direct-child rewrites.
• Recompute a parent schema only when a rewritten child’s schema changes.
• Migrate EliminateCrossJoin to use the helper.
• Preserve existing in-place optimizer schema protection.
• Add tests for schema-changing and schema-preserving child rewrites.

This is an internal optimizer refactor and does not change SQL-visible behavior.

Testing

• cargo fmt --all --check
• cargo test -p datafusion-optimizer

The optimizer tests passed, including unit tests, integration tests, and doctests.
cargo clippy is currently blocked by an unrelated existing dead_code error in
datafusion-physical-expr.