Support thresholds for 2D weights in assertions.

[ruseinov]

Currently we aren't able to use thresholds, because they are designed to work with string/int values as opposed to a json object with two values.

Here's an example of how that might look:

                    - name: polkadotXcm.Attempted
                      attributes:
                        - type: XcmV3TraitsOutcome
                          xcmOutcome: Complete
                          threshold: { refTime: [10, 10], proofSize: [20,20] }
                          value: { refTime: 100000, proofSize: 200 }