Finish work on InitializationEvents

[breedx-splk]

As part of #121, the InitializationEvents class was introduced. It's just a shell/stub at the moment and the work needs to be completed. Logging this issue so that we don't lose sight of this fact.

It also probably doesn't make too much sense to work on it until the Events Working Group has completed most of the spec work it is currently undertaking.

When implementing this, we will also need to take care to handle the fact that some of these initialization events are generated before the SDK is initialized. As a result, the data will need to be buffered in a non-otel structure until the sdk has finished being initialized.