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.
I think you can use this in the constructor:

```
self._app = sgtk.platform.current_bundle()
```

(it's relatively new so wasn't around when the workfiles app was first written!)

That could potentially simplify things a little bit!