Add shared cron :schedule schema validator

lib/oban/plugins/reindexer.ex

@@ -78,7 +78,7 @@ defmodule Oban.Plugins.Reindexer do
       conf: :ok,
       name: :ok,
       indexes: {:list, :string},
-      schedule: {:custom, &validate_schedule/1},
+      schedule: :schedule,
       timeout: :timeout,
       timezone: :timezone
     )
@@ -127,16 +127,6 @@ defmodule Oban.Plugins.Reindexer do
     {:noreply, schedule_reindex(state)}
   end
 
-  # Validation
-
-  defp validate_schedule(schedule) do
-    Expression.parse!(schedule)
-
-    :ok
-  rescue
-    error in [ArgumentError] -> {:error, error.message}
-  end
-
   # Scheduling
 
   defp schedule_reindex(state) do

lib/oban/validation.ex

@@ -110,6 +110,15 @@ defmodule Oban.Validation do
     {:error, "expected #{inspect(key)} to be between #{min}..#{max}, got: #{inspect(val)}"}
   end
 
+  defp validate_type(:schedule, key, val) do
+    Oban.Cron.Expression.parse!(val)
+
+    :ok
+  rescue
+    error in [ArgumentError] ->
+      {:error, "expected #{inspect(key)} to be a valid cron schedule, got: #{error.message}"}
+  end
+
   defp validate_type(:string, key, val) when not is_binary(val) do
     {:error, "expected #{inspect(key)} to be a string, got: #{inspect(val)}"}
   end