[CI][Integration] Add extension-wrapped union to integration data

[pitrou]

Describe the enhancement requested

Just as what was done in the C++ test suite in #50927, we would like to add extension-wrapped union data to the cross-implementation integration datagen, to make sure that this case is handled properly by all tested implementations.

Component(s)

Continuous Integration

[pitrou]

cc @fenfeng9 @paleolimbot @alamb

[pitrou]

Also @tadeja