GroupSpec signature

[d-v-b]

Currently we have

TAttr = TypeVar("TAttr", bound=Mapping[str, Any])
TItem = TypeVar("TItem", bound=Union["GroupSpec", "ArraySpec"])

class GroupSpec(NodeSpecV2, Generic[TAttr, TItem]):
    attributes: TAttr
    members: dict[str, TItem]

This excludes the key space of members from the type domain (the keys can be any strings).

What if we did this instead:

TAttr = TypeVar("TAttr", bound=Mapping[str, Any])
TItem = TypeVar("TItem", bound=Dict[str, Union["GroupSpec", "ArraySpec"]])

class GroupSpec(NodeSpecV2, Generic[TAttr, TItem]):
    attributes: TAttr
    members: TItem

I think this would enable requiring that Zarr groups contain nodes with specific names, e.g.

class MyMembers(TypedDict):
    s0: ArraySpec[ArrayAttrs]
    s1: ArraySpec[ArrayAttrs]

GroupSpec[MyAttributes, MyMembers](...)

But it's not clear to how to make this play nice with pydantic models, since they can't be coerced to Dict[str, blablabla]