Support variadic generics

[dmontagu]

We eventually want to support variadic generics.

There are already some tests for the desired behavior (thanks @caniko).

I'm creating this issue for tracking purposes as I have closed #5351.

[caniko]

Any news on this front?

[dmontagu]

Working on it now. I think there may be some issues with the existing xfailing tests for this stuff, and it's a bit more complicated than I expected, but making progress..

[dmontagu]

I think we'll need to modify pydantic-core to support a tuple schema with a variable-length item in the "middle"; right now, I believe the tuple-positional schema only supports variable-number of "tail" items, but in https://peps.python.org/pep-0646/ it is clearly supported to have any number of specific parameters at the start and/or end of the tuple.

I would propose that we actually unify the tuple-positional and tuple-variable schemas, with a schema roughly along the lines of:

class TupleSchema(TypedDict, total=False):
    type: Required[Literal['tuple']]
    leading_items_schema: List[CoreSchema]
    trailing_items_schema: List[CoreSchema]
    variadic_schema: CoreSchema
    min_variadic_length: int
    max_variadic_length: int
    strict: bool
    ref: str
    metadata: Any
    serialization: IncExSeqOrElseSerSchema