[Lens] Dimension editor should allow fieldless operations

[wylieconlon]

To support the filters aggregation and future calculations like math, we will need the editor to sometimes hide the field selector. When switching to and from the fieldless operation, Lens should remember the last-used field.

Introducing a fieldless operation should probably not affect any of the suggestion logic that we currently apply, such as auto-selecting a field for a function.

[elasticmachine]

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

[flash1293]

When we drop the requirement to remember the last used field, we could implement this with little effort by introducing a hidden virtual field none which hides the field select when selected by choosing a field less operation.

All other interactions would continue in the same way, but it wouldn't switch back to the last used field e.g. when going from "Top values of field1" to "Filters" back to "Top values" - in this case you would have to pick a field again.

@wylieconlon do you have another idea how we could represent this?
@AlonaNadler what do you think about the behavior in this case (switching to filters and back requires you to pick a field again)?

[wylieconlon]

I was expecting that we would implement the various pieces of this in three phases that can be done in parallel:

• The hacky way with filters on "Records", just to show that it works
• Begin hiding the field selector and maybe changing the overall Lens dimension panel logic
• Implement a memory for previously-used dimensions: [Lens] Remember last-used configuration when switching between incompatible configurations #75204

[flash1293]

Makes sense, it seems like we need the first two things for a nice experience to merge the filters PR, then adding the third point as an improvement later (probably after 7.10)