Generate field-based filter helpers

[vst]

This is rather tricky, but the idea is to provide a codec and interpreter for the following:

{
  "_and": [
    {
      "id": {
        "_in": [
          1,
          2,
          3
        ]
      }
    },
    {
      "_not": {
        "name": {
          "_ilike": "%ignore%"
        }
      }
    },
    {
      "createdAt": {
        "_range": {
          "_start": "2012-12-12",
          "_end": "2014-12-12"
        }
      }
    }
  ]
}

The idea is similar to what PostgREST and Hasura provides. There is a trade-off between:

1. An extensible DSL so that user can later provide custom filters, and
2. A powerful DSL that can traverse through relationships.

Another challenge is providing an auth context for filtering: Some users should not be able to filter on certain fields/relationships to avoid information leakage.