BE: Endpoint for updating all custom field values for a resource

[ssangervasi]

The FE for editing custom fields has to do N API calls in order to save changes to N custom fields for a resource, including a mix of DELETE and PUT requests: https://github.com/ethyca/fides/pull/2586/files#r1106236249

This process would be simpler (and could provide better info to the user) if the BE instead had an API for setting all custom field values for a resource in one requests. This would need to clear out any fields that were not included in the request, along with upserting the values that were passed.