Add support for the new v3 matching rules format

https://github.com/pact-foundation/pact-specification/tree/version-3#example

Change from 

```
{
  "matchingRules": {
    "$.body.foo": {}
  }
}

```

to

```
{
  "matchingRules": {
    "body": {
      "$.foo": {}
    }
  }
}

```

This is required so that it can operate with message pacts written by pact-jvm.