fix: add get(key, default) method to Group APIs

[jhamman]

Some downstream applications (xarray.DataTree) were using Group.get. This PR adds that to the Group API for backward compatibility.

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)

[TomAugspurger]

I don't really know how to type this properly, but IMO as a user I'd appreciate the ability to know that .get(key, default) is still AsyncArray | AsyncGroup (or None I guess). So maybe we have DefaultT also be bound to AsyncArray, AsyncGroup?

That said

from typing import reveal_type

foo : dict[str, int] = {}
reveal_type(foo.get("a", "b"))

gives

src/zarr/core/metadata/v3.py:555: note: Revealed type is "Union[builtins.int, builtins.str]"

so nevermind :/ I guess maybe by providing an AsyncGroup as a default, the DefaultT will be AsyncGroup and I'd get what I want.