Consider running in CI a static type checker like `mypy`

The `jaxsim` package is widely type hinted, however at the moment it's only useful while developing for IDE suggestions. It would be great if, slowly, we can start having `mypy` running at least on the `jaxsim.api` package.

The starting point is not that easy. I get ~200 errors.

- https://mypy.readthedocs.io/en/stable/index.html

Start by inspecting the repository's CI configuration and the jaxsim.api package, then run mypy to review the approximately 200 reported errors. Done means mypy runs in CI for at least jaxsim.api with an agreed approach for addressing or handling the existing errors.

Written by the indexing model from the issue text.