[FEATURE] Support resolute sql expression for Lance add column request

### Describe the feature

Currently, `AlterTableAddColumnsRequest` only supports passing a SQL string when adding new columns

```
public class AlterTableAddColumnsRequest {
public static final String JSON_PROPERTY_ID = "id";
@Nullable
private List id = new ArrayList();
public static final String JSON_PROPERTY_NEW_COLUMNS = "new_columns";
@Nonnull
private List newColumns = new ArrayList();

public AlterTableAddColumnsRequest() {
}

public class NewColumnTransform {
public static final String JSON_PROPERTY_NAME = "name";
@Nonnull
private String name;
public static final String JSON_PROPERTY_EXPRESSION = "expression";
@Nonnull
private String expression;

public NewColumnTransform() {
}
}
```

Then SQL expresison can be `CAST(NULL AS int)`, `cast((id + 1) as int)`

### Motivation

_No response_

### Describe the solution

_No response_

### Additional context

_No response_

Locate AlterTableAddColumnsRequest and NewColumnTransform, then trace the Lance add-column request path that consumes their expression field. Confirm how SQL expressions such as CAST(NULL AS int) and cast((id + 1) as int) should be represented and validated; the work is done when those expressions are supported for adding Lance columns.

Written by the indexing model from the issue text.