I've created an SNS topic, and I'd like to subscribe to it with a filter policy that matches a nested attribute. For example, given a message like this:

{
  "foo": {
    "bar": "baz"
  },
  "quux": "vorp"
}

I'd like to match only messages where the bar attribute of foo is equal to baz.

The documentation I've found so far only mentions matching attributes specified at the top level. I'm interested in a nested attribute. For the purposes of this question, let's assume I don't control the structure of the message.