feat: Support default values in `UpdateSchema`

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

```
There is a recent PR to support default values in UpdateSchema, it would be good to port that to iceberg-rs as well: https://github.com/apache/iceberg/commit/602c35a31833f8448734781d71d949ec59abe45b
```
@CTTY Has a comment in https://github.com/apache/iceberg-rust/pull/1172 to support default values in `UpdateSchema`. Here is a reference to the pull request in [java](https://github.com/apache/iceberg/commit/602c35a31833f8448734781d71d949ec59abe45b).

### Describe the solution you'd like

_No response_

### Willingness to contribute

None

Start with the UpdateSchema implementation and read the referenced Java commit 602c35a31833f8448734781d71d949ec59abe45b alongside the discussion in pull request #1172. Define the Rust behavior needed for default values and consider how completion can be verified against the Java reference.

Written by the indexing model from the issue text.