feat: Port SignatureListItem to Rust

[kaylendog]

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[erikjohnston]

Drive-by: FYI you can use #[pyclass(frozen, get_all)] to automatically add #[get] to all fields. There is also a new = "from_fields", but I haven't used it.

[kaylendog]

Thanks! Looks like the new attribute is part of 0.28.0, so would need to bump the version. Best done in a separate PR?