feat(api-events): add data property to events api

[pkanal]

Closes #113
Unblocks #108

Changes

• Adds data property to Event interface that should be used by instrumentation. attributes should only be used to provide additional attributes

Questions

Do we also need to update the emit function here? There was some discussion about making that function take arguments instead of one argument that is the Event interface.

[martinkuba]

I am fine with merging this for now so that we can unblock other work. But we should update to the LogAttributes type once it is brought over here (it is already available in the core JS repo - open-telemetry/opentelemetry-js#3821).