Consider to implement warning for optional fields with default value

[mikir]

Even optional fields can have default values. However, if optional value has default value, it means that this field will be set during construction of Zserio object. Thus, such field will be always present after Zserio object construction which might be misleading for users.

Therefore, it might be good if zserio compiler could fire a warning in case of any optional field with default value.