[Lens] Add ability to copy field names

[rayafratkina]

Describe the feature:
When the user is dealing with a complex schema with long field names, it would be incredibly useful to be able to copy field names.
Copy should work in the field list on the left and in visualization config on the right.

Describe a specific use case for the feature:
A couple of user flows I ran into

• copy field from the left, paste it in the vis as part of creating filters
• copy field used in the filter, paste it into another filter and change something small to get a different fiel
• copy field from the vis config, paste it into the filter bar

cc @ghudgins

[elasticmachine]

Pinging @elastic/kibana-vis-editors (Team:VisEditors)

[stratoula]

Copying field names from the field list is now possible cc @jughosta

[jughosta]

Yes, from the title of a field popover.