As user of the Objects API I want to identify objects by a custom but unique identifier

[joeribekker]

This allows for proper updates on objects that have their own identification property and don't use the UUID given by the Objects API. It can ofcourse be combined from other fields on the client-side.

The identification-field is part of the Objects API Object and can be whatever value as long as it's less than 100 chars and unique in combination with its type. Filtering on this field, in combination with a type, should be allowed.

We can make the field optional, mostly used for sync-operations with data that does not store the UUID of each object.