Allow sorting by computed columns in EuiInMemoryTable

[chandlerprall]

Summary

Fixes #2012.

Previously the actively sorted column was tracked by the column's field value. In computed columns there is no field. This PR changes the tracking to use name, and updates EuiInMemoryTable and EuiBasicTable to accept the target column's field or name, providing backwards compatibility.

I've updated one existing unit test so it actually tests something, and have added four more unit tests to cover the new sort case and check two forms of backwards compatibility. Updated the In-Memory Table - Custom sort values docs section with an example of sorting by a computed field.

Checklist

- [ ] This was checked in mobile
- [ ] This was checked in IE11
- [ ] This was checked in dark mode

• Any props added have proper autodocs
• Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
  - [ ] This was checked against keyboard-only and screenreader scenarios
  - [ ] This required updates to Framer X components

[chrisronline]

I pulled this down, built it locally, copied the files over to my local Kibana, and tried it out and it works great! LGTM!

[chandlerprall]

@pugnascotia applied your suggestions, definitely like testing against the explicit values!

[chandlerprall]

jenkins test this

[chandlerprall]

jenkins test this