Extend json path assertions

[theredfish]

Once https://github.com/besok/jsonpath-rust/issues/40 is fixed, we will be able to extend json path assertions and check if

• [ ] the path exists
  • [ ] also check for false positives/negatives in other assertions. Fail with Unprocessable?
• [ ] the path contains (use JsonPathFinder)
• [ ] the path does not contain

See for some kind of high order function restricting these checks to a Value::Number - or maybe returns a failure with Unprocessable result?

• [ ] the value at path is less than, is greater than or between