[Discover] [Unified Data Table] Improve UX when all columns are removed except those with absolute widths

[davismcphee]

When removing all columns from the Discover data grid except those with absolute pixel widths, the grid looks odd since the remaining columns do not fill the available space, and there is no way to fix it as a user other than removing and re-adding a column so that it resets to auto width:

We should do something to improve the UX here. Possible solutions include:

• We could add a "Reset width" or "Auto width" option to the column menu popover. This would solve the issue of having to remove and re-add a column to reset it back to auto width, but still involves a manual step by the user. In general, a reset width option for columns probably makes sense regardless.
• We could try to be a bit smarter about it to avoid needing a manual step, such as automatically resetting the last column to auto width when all other auto width columns are removed. This seems more convenient for users but involves some assumptions about what the user would want and potentially risks introducing an unexpected behaviour.

[elasticmachine]

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