Switch to using a single event type

Nobody has claimed this yet.

**Issue by [marcoguerri](https://github.com/marcoguerri)**
_Wednesday Jun 17, 2020 at 09:09 GMT_
_Originally opened as https://github.com/facebookincubator/contest/issues/123_

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At the moment we have two type of events, `testevents` and `frameworkevents`. The former are meant to be used when emitted by a `Step`, the latter from anywhere in the framework outside of a `Step`. The intention was to enforce non-null on most of the fields of the `Step`.

Eventually we settled on the fact that this is too restrictive. `FrameworkEvents` are barely used, and lots of places of the framework need to emit events that include a subset of the information of `testevents`.

So, we agree on deprecating `frameworkevents`, and using one single concept of `event`, where fields might be null. In addition, this event should be associated to a "source" in the header (most likely split into `SourceType` and `SourceName`), which will indicate who emitted the event.

Start by locating the testevents and frameworkevents definitions and the Step and framework code that emits them. Trace which fields each event currently requires, then define the scope for one nullable event type with source information. Done means frameworkevents is deprecated and event emission consistently uses the unified concept.

Written by the indexing model from the issue text.