feat: support Fn::ImportValue

Closes #262

Author: theburningmonk

lib/deploy/stepFunctions/compileStateMachines.schema.js

@@ -8,6 +8,12 @@ const arn = Joi.alternatives().try(
   Joi.object().keys({
     'Fn::GetAtt': Joi.array().items(Joi.string()),
   }),
+  Joi.object().keys({
+    'Fn::ImportValue': Joi.alternatives().try(
+      Joi.string(),
+      Joi.object(),
+    ),
+  }),
 );
 
 const definition = Joi.alternatives().try(