Add typing stubs (`.pyi` files) for PyO3-generated Python definitions

Even with https://github.com/channable/opsqueue/issues/8 resolved, type checkers will treat types that are exclusively defined via C bindings as `Any`. Typing stubs would fix this, but PyO3 currently does not generate them (https://github.com/PyO3/pyo3/issues/5137). Manually maintaining them would be annoying because it'd have to be kept in sync with the Rust definitions of the types, but it could still be valuable as a temporary solution.

Start by locating the Rust definitions exposed through PyO3 and checking how the generated Python definitions are currently represented. Review PyO3 issue 5137 and the existing type-checking setup, then define which exported types and signatures the stubs must cover. Done means the stubs describe the relevant bindings without leaving those types as Any and are documented as maintainable alongside the Rust definitions.

Written by the indexing model from the issue text.