[Lens] When dragging a field to the workspace, let user know what will happen after dropping a field

[mbondyra]

Describe the feature: Lens users have little confidence over dragging a field to the main workspace area. The reason for it is that they don't know what will happen if they drop a field there. The questions they might have are:

1. Would some existing configuration be replaced?
2. Where will the field be added, what group?
3. Why, in some cases, you actually cannot drop a field to the workspace?

To make the user experience better, we might design something that would indicate to the user what will happen, maybe in a text form or simplified chart.

[elasticmachine]

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