"There is nothing in the Constitution which precludes a justice a Chapter III court from, in his personal capacity, being appointed to an office involving the performance of administrative or executive functions including functions which are quasi-judicial in their nature. Such an appointment does not involve any impermissible attempt to confer upon a Chapter III court functions which are antithetical to the exercise of judicial power. Indeed, it does not involve the conferring of any functions at all on such a court."