Add a data transformer to transform a json to Author.

lib/decentralised_book_index/sync/data_transformers/author_transformer.ex

@@ -0,0 +1,14 @@
+defmodule DecentralisedBookIndex.Sync.DataTransformers.AuthorTransformer do
+
+  alias DecentralisedBookIndex.Metadata.Author
+
+  def from_json(json_body) do
+    author = %Author{
+      id: get_in(json_body, ["data", "id"]),
+      name: get_in(json_body, ["data", "attributes", "name"]),
+      description: get_in(json_body, ["data", "attributes", "description"]),
+    }
+
+    {:ok, author}
+  end
+end