Refactor micropub specific things out to a slice

slices/micropub/validation/posts/book_contract.rb

@@ -0,0 +1,18 @@
+module Micropub
+  module Validation
+    module Posts
+      class BookContract < Dry::Validation::Contract
+        params do
+          required(:name).maybe(:string)
+          required(:content).filled(:string)
+          required(:category).array(:string)
+          required(:published_at).maybe(:time)
+          required(:slug).filled(:string)
+          required(:post_type).value(included_in?: %w[book])
+          required(:book_author).maybe(:string)
+          required(:book_status).value(included_in?: %w[to-read reading read finished])
+        end
+      end
+    end
+  end
+end