Add backlight PV to bluesky

I23 uses a selection that picks whether to use a backlight and which backlight with a PV. It would be helpful to add it to Bluesky as it'll be helpful for sample centering.

`BL23I-DI-OAV-01:CAM:MP:SELECT`

## Acceptance Criteria
* Errors won't occur when restarting bluesky server after implementation on any beamline.

Start by locating the Bluesky device definitions and the existing I23 backlight integration points for PV `BL23I-DI-OAV-01:CAM:MP:SELECT`. Check how device setup behaves across beamlines, then verify that restarting the Bluesky server produces no errors after the PV is added.

Written by the indexing model from the issue text.