add support for dictionary field type as pyarrow map array

[mae5357]

issue

Need support for python's dictionary type to convert pydantic object to pyarrow.

e.g.

MyModel(BaseModel):
    foo: Dict[str, str]

get_pyarrow_schema(MyModel)

change

If a pydantic field is defined as a Dict, convert to pyarrow.MapType [definition].

MapType is the closest pyarrow data type to the Dict type. However, it will convert to a tuple upon deserialization (from pyarrow to vanilla python).
https://arrow.apache.org/docs/python/data.html#map-arrays

note: this is a rather naive approach. open to suggestions... by default, pyarrow will try to use pyarrow.StructScalar. However, this will not work if inferring the schema of a Dict of unknown size.

test

assert the additional logic makes sense. assert the new test case passes.

[simw]

Thank you, looks great!

[simw]

Included in release 0.1.3