[Lens] Frame allows visualizations to have multiple types of layers with no datasource

[wylieconlon]

This is technical work that blocks work on the thresholds feature and annotations feature.

This isn't an official design, but it shows that we need users to be able to add additional features to their charts, which might resemble the UI to add layers:

Each of these types of layers can have a custom UI, and they don't need to be backed by a datasource. Thresholds can be stored entirely within the visualization state:

Even though these layers don't have a datasource, it is possible for the visualization to load data asynchronously. This will support the annotations feature. For example, the visualization can change the expression by adding a fetching step: lens_xy_chart annotations={fetchAnnotations}

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[flash1293]

Done for reference lines