Determine configuration strategy for new events

[rgraber]

A/C

• An ADR detailing how new events will be rolled out to the community

Since #249, new events can be added to the event bus via configuration only (no code changes). Some options for how we would roll out new events:

1. All events are off by default. New events are added to the configuration map in the producing service's configuration file with enabled=False so people know what events are available
2. All events are on by default. New events are added to the configuration map in the config with enabled=True
3. All new events start disabled by default, but are updated to be enabled by default as part of a planned release (with the change noted in the release notes)

[robrap]

Note: Option 3 uses enabled as a temporary rollout toggle. If we use this route (which I like), we'd need to also understand how to get the best of https://edx.readthedocs.io/projects/edx-toggles/en/latest/how_to/documenting_new_feature_toggles.html. Or, maybe we should quite literally be using the toggle annotations for each of these?