chore: add generic type hints for remaining modules + mark project as typed.

[nikshankarnoble]

Problem

Completion of the work started on #377 of adding generic type hints to the Gazu codebase - more information can be found on that PR and linked discussion.

Solution

This PR adds type hints to the remaining modules in the API, and finally adds the py.typed file indicating that the module has type annotations.

As with the last PRs, along the way I've also made some minor fixes/additions to docstrings, in cases where I didn't find the types or behaviours of the arguments immediately obvious - or there was incorrect/missing info in the docstrings.

And in or two instances have added normalize_model_parameter() calls to parameters so that their functionality matches the previous docstring.

I've also standardised model parameter docstrings from dict / ID to str / dict in a couple of places so they're all the same.