Lookup table for extended attribute options

[car031]

For a Document Template we can add an arbitrary number of custom text fields. If we want to handle an enumeration of fixed values we need to put all the option in a comma-separated values string.

When the options are more than 10 it is cumbersome to add all of them in the standard textbox so it would be better to connect the drop-down list to a specialized lookup table.
This can be implemented by using a dedicated table key-value or we can reuse the Generics.