Add runtime option to disable expunging

[VanceLongwill]

For local debugging/dev environments etc there should be a way to disable expunging at runtime. This could be:

• An env var
• Some kind of init/guard similar to slog
• Something else

When expunging is disabled the Expunge trait should return the original value verbatim, without redacting any values.

impl Expunge for T {
    fn expunge(self) -> Self
    where
        Self: Sized,
    {
        if expunge_disabled {
           return self;
        }
        // if not, then proceed as normal
        ...
    }
}

TODO:

• Determine how the runtime state will be fetched from within the Expunge trait impl