iOS bindings ErgoBoxes.get uses DataInputPtr

Curious why a `DataInputPtr` is being used instead of an `ErgoBoxPtr`: https://github.com/ergoplatform/sigma-rust/blob/develop/bindings/ergo-lib-ios/Sources/ErgoLib/ErgoBox.swift#L514 when the C header has this as the method signature:

```
struct ReturnOption ergo_lib_ergo_boxes_get(ConstErgoBoxesPtr collection_ptr,
uintptr_t index,
ErgoBoxPtr *element_ptr_out);
```

Start with bindings/ergo-lib-ios/Sources/ErgoLib/ErgoBox.swift at the referenced line and compare its DataInputPtr usage with the ergo_lib_ergo_boxes_get declaration in the C header. Confirm which pointer type the Swift binding should use, update the binding if it does not match, and validate the iOS bindings build.

Written by the indexing model from the issue text.