Bug with expected format in `included` data

[doomspork]

I need to flatten some JSON:API data down in another project so I wanted to look at whether I could leverage the JSONAPI.Utils.DataToParams.process/1 functionality. When I passed in our JSON:API data I got an error:

iex> JSONAPI.Utils.DataToParams.process(file)
** (FunctionClauseError) no function clause matching in anonymous fn/2 in JSONAPI.Utils.DataToParams.process_included/1

    The following arguments were given to anonymous fn/2 in JSONAPI.Utils.DataToParams.process_included/1:

        # 1
        %{
          "attributes" => %{
            "damaged" => nil,
            "expiresAt" => nil,
            "insertedAt" => "2021-11-30T19:15:35Z",
            "integrationMetadata" => %{"customData1" => "__MWJ1071102-MWC1402073"},
            "lotNumber" => nil,
            "quantity" => "300",
            "unit" => "ea",
            "updatedAt" => "2021-11-30T19:15:35Z"
          },
          "id" => "0e4c7852-0ac3-4ee9-8d1d-841ec171976b",
          "relationships" => %{
            "item" => %{
              "data" => %{
                "id" => "8031cf98-89f8-4a1c-a787-b74f6e76a44c",
                "type" => "item"
              },
              "links" => %{"related" => "/items/8031cf98-89f8-4a1c-a787-b74f6e76a44c"}
            }
          },
          "type" => "lineItem"
        }

When I went to the line specified to see what was expected I noticed that we expected a "data" key in each included resources: https://github.com/jeregrine/jsonapi/blob/master/lib/jsonapi/utils/data_to_params.ex#L97 and that's reflected in our tests: https://github.com/jeregrine/jsonapi/blob/master/test/utils/data_to_params_test.exs#L96-L132

 test "processes single includes" do
    incoming = %{
      "data" => %{
        "id" => "1",
        "type" => "user",
        "attributes" => %{
          "name" => "Jerome"
        }
      },
      "included" => [
        %{
          "data" => %{
            "attributes" => %{
              "name" => "Tara"
            },
            "id" => "234",
            "type" => "friend"
          }
        }
      ]
    }

    result = JSONAPI.Utils.DataToParams.process(incoming)

    assert result == %{
             "friend" => [
               %{
                 "name" => "Tara",
                 "id" => "234",
                 "type" => "friend"
               }
             ],
             "id" => "1",
             "type" => "user",
             "name" => "Jerome"
           }
  end

I believe this to be incorrect. If we look at https://jsonapi.org they do not have the "data" key in each of the included resources:

 "included": [{
    "type": "people",
    "id": "9",
    "attributes": {
      "firstName": "Dan",
      "lastName": "Gebhardt",
      "twitter": "dgeb"
    },
    "links": {
      "self": "http://example.com/people/9"
    }
  }, {
    "type": "comments",
    "id": "5",
    "attributes": {
      "body": "First!"
    },
    "relationships": {
      "author": {
        "data": { "type": "people", "id": "2" }
      }
    },
    "links": {
      "self": "http://example.com/comments/5"
    }
  }

If someone else can verify this is in fact a bug I can put together a PR to correct this.

[doomspork]

I found potentially 2 other issues with our parsing:

1. Parsing relationships also requires "data" but per the spec it is not required: https://jsonapi.org/format/#document-resource-object-relationships. In our case some of our associated data only includes a "link" key.
2. We don't appear to handle nested relationships which is also supported by the spec: https://jsonapi.org/format/#document-resource-objects.

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