Add more typing to zarr.group

[dstansby]

This finishes most of the typing for zarr.group. I think (at least for now) typing all the kwargs as Any is the best we can do?

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[jhamman]

I think (at least for now) typing all the kwargs as Any is the best we can do?

I think that is right. Once we have all of these convenience methods implemented, I we can probably do a better job typing the implementations.