Deepcopy incorrectly sets the 'active' oneof field to last field in structure (breaking which_one_of)

### Summary

When calling deepcopy on a proto, the oneOf field state gets overwritten.

### Reproduction Steps

1. Create a simple proto with two one_of fields.
2. set the first field
3. Call which_one_of which will return the first field
4. Call deepcopy on the proto
5. check which_one_of again.
6. The second field is returned.

The implementation of __deepcopy__ does this:
```
for name in self._betterproto.sorted_field_names:
value = self.__raw_get(name)
if value is not PLACEHOLDER:
kwargs[name] = deepcopy(value)
```
And of course, setattr sets the current group by field, which overwrites this.

one option is to simply make deepcopy one_of aware, using oneof_field_by_group, default initializing the nonset fields and adding them to a set, and then only iterating over the names that aren't in the set using the overall loop above.

### Expected Results

We preserve the correct one_of field

### Actual Results

The second one_of field is set.

### System Information

libprotoc 3.21.12
Python 3.11.9
Name: betterproto
Version: 2.0.0b6
Summary: A better Protobuf / gRPC generator & library
Home-page: https://github.com/danielgtaylor/python-betterproto
Author: Daniel G. Taylor
Author-email: danielgtaylor@gmail.com
License: MIT
Location: /usr/local/lib/python3.11/site-packages
Requires: grpclib, python-dateutil
Required-by:

### Checklist

- [X] I have searched the issues for duplicates.
- [X] I have shown the entire traceback, if possible.
- [X] I have verified this issue occurs on the latest prelease of betterproto which can be installed using `pip install -U --pre betterproto`, if possible.

issue に示されている __deepcopy__ の実装から始め、oneof_field_by_group がアクティブなフィールドをどのように表現しているかを調べます。2 つの one_of フィールドを含む proto でこのケースを再現し、コピー前に選択されていたフィールドが deepcopy によって保持されること、またその後 which_one_of が同じフィールドを返すことを確認します。

索引モデルが issue の本文から書いたものです。