Implement the NullPatternGenerator

[rtuck99]

This PR implements a NullPatternGenerator.

See related PR

• Prevent unit tests creating > 2GB of temp files DiamondLightSource/dodal#1270

The motivation for this is that usage of SimBlobDetector in dodal unit tests was generating several GB of temporary files. This was traced back to the PatternGenerator which persists the output to a file. As the usage of the SimBlobDetector in the tests did not require examination of the detector output (only the ability to exercise it), a NullPatternGenerator was created and @DominicOram suggested it should be made part of ophyd-async.

[coretl]

I'm not convinced I really want to support a detector that lies about writing files, especially in the sim package. Could it stay in dodal?

Alternatively, you can tell pytest to clean up after itself faster:

• https://docs.pytest.org/en/stable/reference/reference.html#confval-tmp_path_retention_policy
• https://docs.pytest.org/en/stable/reference/reference.html#confval-tmp_path_retention_count

Or you could clean up the tmp_path yourself in the fixture for the detector

[DominicOram]

If @coretl doesn't think this is useful then I'm happy to just have it in dodal, apologies.

[coretl]

No problem, thanks for thinking of pushing it up!