Encode oneOfs as Python unions for reliable handling.

[heimalne]

Currently, betterproto encodes protobuf oneOfs as a attributes of the enclosing class with one attribute per oneOf alternative. This leads to an akward runtime-encoding, where all attributes get an object with dummy data and the actual oneOf has non-dummy data. To distinguish those cases we need helper functions like serialized_on_wire() or which_on_of(), and the mypy type checker can't help us to ensure we've covered all alternatives. A better encoding would be Python unions (a.k.a Choice types), which get full exhaustiveness-support mypy support with Python 3.11 (https://docs.python.org/3.11/library/typing.html#typing.assert_never) and backwards compatible workaround for older Python versions (https://hakibenita.com/python-mypy-exhaustive-checking)

Would this other union encoding be suitable for betterproto?

[Gobot1234]

Duplicate of #358