Support for multiple items received in a single field

[astropenguin]

This issue will introduce a new type hint, Multiple[T], an alias of dict[str, T] with a dedicated tag, Tag.MULTIPLE. Combined with other type hints (e.g. Multiple[Data[Any]]), it will enable the corresponding field receive a dictionary of data, indexes, or attributes. For example, the attrs field in the following code will accept a dictionary of any values which will be stored in attrs in a DataFrame.

from dataclasses import dataclass, field
from pandas_dataclasses import AsFrame, Attr, Data, Index, Multiple


@dataclass
class Weather(AsFrame):
    """Weather information."""

    year: Index[int]
    month: Index[int]
    temp: Data[float]
    wind: Data[float]
    attrs: Multiple[Attr[Any]] = field(default_factory=dict)