Support @sensitive trait

[robin-aws]

This looks at least partially implemented - Dafny and Python both omit it from generated to-string functions.

It's heavily used in downstream projects but mainly for internal documentation of intent, so I'm removing the DANGER warning about the trait but tracking the gap with this issue instead.

[robin-aws]

Switching to feature request, since we suppressed the warnings in downstream projects instead of removing the warning here.