FileAssay - access_control

I don't see anything that aligns with the ACR model's access policy concept. Was that ACR approach determined to be inappropriate? 

in the d3b255 branch, [access_type ](https://github.com/include-dcc/include-access-model/blob/d3b-2559-add-files-and-assay/src/include_access_model/schema/include_access_model.yaml#L1049) is a string. It seems like it should at least be an enumeration. 