Add Chain.when(boolean, Action) variant for conditionally adding handlers at chain build time

[ldaley]

We currently have this method: https://ratpack.io/manual/current/api/ratpack/handling/Chain.html#when-ratpack.func.Predicate-ratpack.func.Action-

It allows you to conditionally route based on request time decisions. This ticket would add the following to chain:

default Chain when(boolean test, Action<? super Chain> action) throws Exception {
    if (test) {
      action.execute(this);
    }
    return this;
}

This would effectively be sugar for conditionally adding to the chain at “build time”.