Allow scripting to support dynamic entity value matches

[rozele]

For example, if a test includes text: "show me my schedule a week from tomorrow", likely we'll want to assert that "a week from tomorrow" is matched as a date entity, but we may also want to check the entity value assigned to that date.

Given that this would vary depending on the time the tests were run, we could allow users to specify a script that would calculate the expected value. E.g.,

DateTimeOffset.Now.Date.AddDays(8)

We could embed these as strings within the expected JSON, e.g.:

{
  "text": "show me my schedule for a week from tomorrow",
  "intent": "Calendar",
  "entities": [
     {
        "entityType": "date",
        "matchText": "a week from tomorrow",
        "entityValue": "csharp(DateTimeOffset.Now.Date.AddDays(8))"
     }
  ]
}

There's still a slight issue in that the time the tests are run may differ from the time the compare command is run, but it alleviates the issue somewhat. We could also consider adding globals that could be included as well, e.g., if we could grab the test run "timestamp" context. In that case, maybe we could add a context object to each test result:

{
  "text": "show me my schedule for a week from tomorrow",
  "intent": "Calendar",
  "entities": [ ... ],
  "globals": {
    "timestamp": "1970-01-01T00:00:00Z"
  }

And we could refer to these globals in the expected output:

{
  "text": "show me my schedule for a week from tomorrow",
  "intent": "Calendar",
  "entities": [
     {
        "entityType": "date",
        "matchText": "a week from tomorrow",
        "entityValue": "csharp(DateTimeOffset.Parse(timestamp).AddDays(8))"
     }
  ]
}