Implement Python unions (#2427)

* Add initial implementation of unions with very broken symbol provider

* Add support for creating new unions in Python

* Generate getters and static methods for unions

* Allow to compile misc model

Signed-off-by: Bigo <1781140+crisidev@users.noreply.github.com>

* Doesn't work

* Now it works

* Simplify code a little

* Remove leftover from the many tries I did

* Finally fixed model generation with unions

* Fix wrong import

* Update to reflect changes in decorators

* Remove debugging output

* Simplify symbol provider

* Follow PR suggestions

* Remove union operation from python example

* Return `PyUnionMarker` for wrapped type in `IntoPy` impl

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Signed-off-by: Bigo <1781140+crisidev@users.noreply.github.com>
Co-authored-by: Burak Varlı <burakvar@amazon.co.uk>