Is your feature request related to a problem? Please describe.

Nowadays many popular OpenAPI specifications describe objects using oneOf / anyOf / allOf keywords. Regardless if this was a good or a bad idea, the OpenAPI generator should support this to avoid having to resort to manual fixes in build chains.

Describe the solution you'd like

Full support for these keywords in all generators.

Describe alternatives you've considered

Right now the only feasible workaround is to manually remove all of the methods that use these schema combine keywords or to implement them by hand in the generated code. This is obviously not ideal.

Additional context

This has been reported in many occassions before: #10450, #10352, #9858 , #9684, #9443, #2906, #5381, #8647, #8860, #9601, #9981, #9305, #9301, #9163, #8860, and many more.