IDL compiler allow optional key when nested in struct

[AlexisPotvinARA]

The IDL compiler allows a value to be an optional key when the optional annotation is nested inside a struct. However, this behavior is not consistent with the specification in 7.2.2.4.4.4.8 Key Members, which states that "Key members shall never be optional, and they shall always have their 'must understand' bit set." To resolve this discrepancy, the IDL compiler and type object validation code should block this behavior.

Here is an example data structure that can be used to validate the error.

struct Plate {
    string number;
    @optional string state;
};
struct Car {
    @key Plate licensePlate;
    string color;
};