Create OAV device

Create a device to mimick the OAV. Will be very useful for testing the python-artemis OAV [294](https://github.com/DiamondLightSource/python-artemis/issues/294).

## Acceptance Criteria
* The device needs to be the bare minimum required for Artemis/GDA to be able to trigger an edge detection. This shouldn't require very much complicated logic. ~~The data coming back from the OAV should be representative of realistic data but again, not complex~~
* **EDIT** this ticket won't deal with making the data realistic. As long as the PVs are there and used in s03, we can write system tests for the OAV. Realistic data should be another issue.