refactor: unify ColumnSource::Add and AddStructConstant using Literal #2699

follow up from #2668

The ColumnSource enum in RecordBatchTransformer currently has two separate variants for constant columns:

Add {
    target_type: DataType,
    value: Option<PrimitiveLiteral>,
},
AddStructConstant {
    fields: Fields,
    child_values: Vec<Option<PrimitiveLiteral>>,
},

AddStructConstant was added in #2668 for the _partition metadata column because Add only supports primitive values. However, iceberg-rust already has Literal::Struct in crates/iceberg/src/spec/values/literal.rs which can represent struct values natively.

Proposed refactoring

1. Change ColumnSource::Add to use Option instead of Option:
   Add {
   target_type: DataType,
   value: Option,
   },
2. Remove ColumnSource::AddStructConstant — struct constants become a Literal::Struct inside Add.
3. Update create_column (and create_struct_column) to handle Literal::Struct by recursively constructing child arrays, similar to how AddStructConstant works today.
4. This also opens the door for constant_fields: HashMap<i32, Datum> to eventually support struct-valued constants (e.g., if Datum gains a struct variant), which would allow the _partition column to use the same constant_fields path as _file and identity-partitioned columns.

Benefits

• Eliminates a special-case enum variant
• Aligns the constant-column infrastructure with iceberg-rust's existing Literal type system
• Makes future metadata columns or complex-typed constants straightforward to add