For #25787 - Display extra info for entities when selecting work area.

Added ability to display extra information for entities in the select work area ui in a similar fashion to the existing functionality for tasks.

For example, you can now specify the following in the environment configuration:

```
sg_entity_type_extra_display_fields: {"Shot":["sg_sequence.Sequence.code"]}
```

which will then result in the Sequence name being shown below the Shot name for each shot when changing work area.  This information is also displayed in the current work area section of the main File manager UI.
So extra_fields is a dict first when you get it, then it becomes a list when you reassign it. Maybe it could be two different variables just for clarity?