[Epic:`.NET`] Consider what the default value for OptionSet fields that are marked as required (also applies for the NodeJS generation) is

Currently there is no place in the UI for the user to specify the default value for entity fields that are marked as required fields. The default value is used for example when generating a DB migration that adds the field as a not nullable field.

This is true for all field types, but it is particularly important in OptionSet (enum) field types, where the user should be given the choice of a default value.

For now, in the dotnet DSG, we default to the first member of the enum until we decide otherwise.