Support matching on JSON keys only

[vmx]

I came across the following use case. I have a JSON structure where the keys of an object are identifiers and the values are some additional information. The object might contain any of the identifiers. I now want to query for all documents that have a specific identifier. That's currently not possible.

Here's an example to make it more concrete:

{
  "outage": {
    "030": "Berlin",
    "089": "Munich"
  }
}

I would now want to query for all outages that contain "089", but I don't care about the actual value.