Fully type module (missing annotations)

This is an issue to track `openpack`'s missing type annotations'.
The package is marked as `py.typed`, but all modules are missing annotations for parameters and return types. This will lead to type errors downstream for any user of strict type checking whenever using any of this package's methods.

Refs:
- https://github.com/jaraco/skeleton/issues/98
- https://github.com/jaraco/skeleton/pull/136#discussion_r1714057809
- https://github.com/jaraco/skeleton/issues/143

Start by reviewing the package's modules and the linked skeleton issue and pull request discussion to understand the expected annotation scope. Add parameter and return type annotations throughout openpack, then verify that strict type checking no longer reports downstream errors for the package.

Written by the indexing model from the issue text.