Make assay/experiment parameters optional that will added once at the dataset level

[anngvu]

Per discussion with @cconrad8, we can save contributors time for file-level annotations for ones that correspond to assay/experiment-level annotations that shouldn't vary over files in the entire dataset, such as Platform and LibraryPrepKit, as opposed to something like PatientID.

To still allow the same type of file search in the portal, when exported the files will need to inherit these values.