[Lens] Do not persist default labels

[flash1293]

Right now we are creating a default label based on field and other information if a new column gets created.
This can cause consistency issues in some cases (like with custom field names #83512) and in general makes transition code more complex because for every transition the "label" prop has to be adjusted.

It's a larger refactoring because it touches a lot of places, but by not storing the default label anywhere and instead requiring code actually acting on the label to generate them using getDefaultLabel of the operation definition, we can reduce the bug surface, fix the consistency issues for custom field names defined on the index pattern and probably simplify the code a little as well.

To keep PRs manageable, I would like to keep this separate from the currently ongoing referencial operation work.

[elasticmachine]

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