[python-nextgen] support constructor with position argument in oneOf/anyOf model

[wing328]

• support constructor with position argument in oneOf/anyOf model
  e.g. Pig(actual_instance=small_pig) vs Pig(small_pig)
• add tests

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FYI @taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11) @tomplus (2018/10) @arun-nalla (2019/11) @spacether (2019/11) @krjakbrjak (2023/02)

[tomplus]

lgtm