Add Time, Now, and Duration expression types

[bkeepers]

Enable a feature at a specific time:

{
  "GreaterThan": [
    { "Now": [] },
    { "Time": "2023-03-26 13:00:00Z" }
  ]
}

Give an actor a 14 day trial:

{
  "LessThan": [
    { "Property": ["created_at"] },
    {
      "Plus": [
        { "Now": [] },
        { "Duration": [14, "days"] }
      ]
    }
  ]
}

Note: Plus isn't implemented yet, but it's on my list

[bkeepers]

Review and updates in #715