Stop skipping private fields for attrs classes

[Tinche]

Description

Continuing our discussion from Bluesky, I would really like for msgspec to stop skipping fields prefixed with _ for attrs classes. This would make attrs classes consistent with dataclasses, and more importantly - enable handling APIs that require field names that start with underscores, which I find to not be super common, but definitely an important use case.

If this is simple, maybe I could contribute this?

[ipseitas]

You can use field to define a custom name mapping between the underscore-prefixed API keys and Struct attributes: https://jcristharif.com/msgspec/api.html#msgspec.field