Makes it possible to have a key that points to a list and assert that a
certain object is present in the list. All keys have to be present and
values have to match. The objects in the source list may have additional
fields.

example:
```
  contains:  { 'nodes.$master.plugins': { name: ingest-attachment }  }
```

Backported from elastic#30874 (e8b8d11)

Co-authored-by: Alpar Torok <torokalpar@gmail.com>