Investigate better ways to handle the typing of `baseline.tags.detections`

[cmcginley-splunk]

• Right now, we have a type for this of List[Union[Detection,str]]
• This is not ideal, as we would rather have ti just be a list of Detections
• However, baselines reference detections and detection reference baselines, so there's a bit of a chicken/egg issue
• We should look into other strategies here