chore(sdk.v2): use Annotated rather than Union hack

[judahrand]

Description of your changes:
This PR deals with the TODO comment by adding typing_extensions as a dependency (it was already required via pydantic) and either importing from standard lib or typing_extensions.

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[chensun]

Thanks for the change. Please add an entry in release note.

[chensun]

Should add this in setup.py as well, we can't rely on an indirect dependency from pydantic.
Also this better be conditioned by a Python version, 3.9 I think?
See dataclass for example:

pipelines/sdk/python/setup.py

Lines 53 to 54 in 55a2fb5

	# Standard library backports
	'dataclasses;python_version<"3.7"',

[chensun]

/ok-to-test

[chensun]

/lgtm
/approve

Thanks!

[google-oss-robot]

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This pull-request has been approved by: chensun

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