Function realize_config_expr 

pub fn realize_config_expr(
    cx: &mut Cx,
    broker: &ReadEvalBroker,
    opt_in: Option<&HostConfigEvalOptIn>,
    expr: Expr,
) -> Result<Expr>

Realizes explicit config eval nodes in a decoded config expression.

With no opt-in, the expression is returned unchanged and no node is parsed or evaluated. With opt-in, config/eval entries in maps are admitted through the broker and their map result is merged into that map. A list headed by config/eval in value position is replaced by its admitted result.