Implement fixtures for running again oak-segment and/or oak-segment-next

We need fixtures to run UTs / ITs against either or both segment implementations oak-segment and oak-segment-next.

Ideally we can enable them individually through e.g. environment variables. A standard build would run against oak-segment so not to affect others. oak-segment-next could be enabled on request locally or for the CI.
Once we deprecate oak-segment we would switch the default fixture to oak-segment-next.