feat(api): support filtering items by item class

[fallenoak]

This PR introduces the ability to filter items by one or more item classes.

For example:

query ExampleQuery {
  items(searchQuery: "torment", itemClassIds: [2, 3]) {
    results {
      id, name, itemClass { id, name }
    }
  }
}

returns:

{
  "data": {
    "items": {
      "results": [
        {
          "id": 25806,
          "name": "Nethekurse's Rod of Torment",
          "itemClass": {
            "id": 2,
            "name": "Weapon"
          }
        },
        {
          "id": 39291,
          "name": "Torment of the Banished",
          "itemClass": {
            "id": 2,
            "name": "Weapon"
          }
        }
      ]
    }
  }
}

The PR introduces the notion of filters separate from search: search permits entering an entity name for a partial string match (as it already does today), while filters support type-and-field-specific equality checks. The collection level entities in the root of the schema would support both searchQuery and filters (assuming the entity type is worth filtering).

[timkurvers]

Awesome, left some small comments.