RFE: API for renaming struct fields

Background

ParseCallbacks::enum_variant_name is already useful for renaming enum variants. I'm using it to ensure that generated bindings fit Rust conventions. However, I'd like to also rename struct fields, and there doesn't seem to be a callback for that.

Proposal

Add fn struct_field_name to ParseCallbacks with the following signature:

fn struct_field_name(
    &self,
    struct_name: Option<&str>,
    original_field_name: &str,
) -> Option<String>

This works in a similar manner to ParseCallbacks::enum_variant_name; if the user chooses to return None, the name isn't changed, but if the user chooses to return Some(string), then the field is renamed to string.

Locate ParseCallbacks and read the existing enum_variant_name callback and its implementation path first. Add the proposed struct_field_name entry point with matching None/Some behavior, then verify that generated struct fields use the returned name without changing the default.