Improve Discover table column configuration controls

[tbragin]

At the moment, the controls to configure Discover table columns are hard to find:

• user has to expand a row
• and realize that they have to click the table icon (that looks like a book to me), to add a column to the table

A more standard control typically has a top-level button that says "configure table columns" or something like that, that has a selector that allows user to pick from a set of available fields/columns.

cc: @cjcenizal

[Bargs]

There's also the "add" button when you hover over the fields in the sidebar, but that's also not very obvious.

[Bargs]

We've encountered users who don't even know this functionality exists. There have to be some small tweaks we could make to vastly improve discoverability here.

[kertal]

I think at the time this issue was written, Lens didn't exist. Since Lens is on the table, we've got another plugin with a sidebar offering fields to add to the data table. So I think this of the primary way to add fields to the table, also I think the icon in the meantime when expanding a document has improved:

We're also currently aligning Lens & Discover with a unified field list, this will also add drag'n'drop to Discover #137779 Therefore I'm closing this issue