Re-enable validation of protobuf unions in `ContainerInfo`

In MESOS-9740, we disabled protobuf union validation for `ContainerInfo` messages, since it was discovered that frameworks generating invalid protobuf of this kind currently exist in the wild.

However, that is somewhat unsatisfactory since it re-enables the issue originally described in MESOS-6874, i.e. Mesos not rejecting tasks where the `ContainerInfo` was accidentally malformed.

Ideally, we should implement a metric counting the number of tasks with malformed `ContainerInfo`s and re-enable validation after an approprate warning period has passed.