Allow setting of custom fields in REST API to work if the Setting Name instead of Setting ID is provided

For example, when setting a Sales Order Line custom field, it would be nice if we could just provide the following DTO on /SalesOrders/{InvoiceID} POST or PATCH operations:

{ "Lines": 
   [{ "InvoiceLineID": "18911c09e80c4ea2a2e9",  
       "CustomFieldValues": 
       [ {"SettingName": "Special Handling Instructions", "Contents": "Don't drop it."} ]
    ]}
}

Instead, we currently must provide the SettingID:

{ "Lines": 
 [{ "InvoiceLineID": "18911c09e80c4ea2a2e9",  
 "CustomFieldValues": 
   [ {"SettingID":"1ae102b94dc54dfc8a45", "SettingName": "Special Handling Instructions", "Contents": "Don't drop it."} ]
 }]
}

Note - we'd need to throw an exception if the setting is ambiguous by SettingName alone - but 99% of the time customers won't have custom fields with the same name.

Also, we should not just do this in sales orders - we should be consistent and do this everywhere that implements IJiwaDTODeserialisable and has custom fields.