Add ImagesDbIds to observationUnits spec

[MFlores2021]

[npanitz]

This might be worth revisiting. I added the plotImageDbIds to our breedbase brapi/v2/observationunits endpoint but it might be worth it to add this to the standard. The observation units endpoint is a nice aggregation of data from other endpoints but organized into trials. I think the image ids for uploaded images for a given observation unit are worth including as a field for the observation units endpoint in BrAPI v2.1