feat: Let nested properties be nullable when using schema helpers

[edgarrmondragon]

Feature scope

Configuration (settings parsing, validation, etc.)

Description

For example, currently

>>> from singer_sdk import typing as th 
>>> th.ArrayType(th.StringType()).to_dict()
{'type': 'array', 'items': {'type': ['string']}}

Ideally, something like this would be supported

>>> from singer_sdk import typing as th 
>>> th.ArrayType(th.StringType(nullable=True)).to_dict()
{'type': 'array', 'items': {'type': ['string', 'null']}}

Workaround

>>> from singer_sdk import typing as th 
>>> th.ArrayType(th.CustomType({"type": ["string", "null"]})).to_dict()
{'type': 'array', 'items': {'type': ['string', 'null']}}