[Discover] Use custom field labels consistently in Discover

[davismcphee]

While testing data view field editing, I noticed we don't use custom field labels consistently in Discover which can lead to confusion for users. I didn't track down everywhere where we fail to use custom field labels, so that work would need to be done as part of this issue, but I can provide a couple examples of what I mean to start.

When saving a data view field with a custom label, the save toast does not use the custom label:

When toggling a column in the table from the expanded document view, the add/remove toast does not use the custom label:

Ideally we would use custom field labels consistently across Discover to prevent confusion.

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[osmanis]

Same goes for the query bar: