Feature Request: Support placeholders for Map state

Currently I am not able to pass execution input to a Map state. If I try to pass execution input to map state it throws 'Object of type 'ExecutionInput' is not JSON serializable' . So is it possible to add palceholder support to Map state.

```
input_data_check_map_step= steps.states.Map(state_id='input_data_check_map',
iterator=input_data_check_lambda_task,
input_path=execution_input,
items_path=execution_input['input_data_dirs'],
parameters={'input_dir':'$.Map.Item.Value','fcst_date':"$.fcst_date"},
max_concurrency=5,
comment='Run input data checks in dynamic parllel state',
result_path='$.InputDataCheckRes')
```