### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

Currently, when passing `$$.Execution.StartTime` as a parameter to a SFN lambda, it is coming in in the format `HH:MM:SS.###`.

### Expected Behavior

`$$.Execution.StartTime` should be an ISO 8601 datetime string.

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

This is run in a docker compose file.

Step function definition is

```json
{
  "Comment": "Sample SFN",
  "StartAt": "SampleLambda",
  "States": {    
    "SampleLambda": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke",
      "End": true,
      "Parameters": {
        "FunctionName": "Sample-Function",
        "Payload": {
          "data": {
            "startDateTime.$": "$$.Execution.StartTime"
          }
        }
      }
    }
  }
}
```

### Environment

```markdown
- OS: Mac OS Ventura 13.6.1
- LocalStack:
LocalStack version: 3.0.1.dev
LocalStack build date: 2023-11-20
LocalStack build git hash: 3e32b438
```


### Anything else?

Thank you so much for your hard work.
self.status = status
        self.stop_date = datetime.datetime.now()
    def describe(self) -> DescribeMapRunOutput:
        describe_output = DescribeMapRunOutput(
            mapRunArn=self.map_run_arn,
            executionArn=self.execution_arn,
            status=self.status,
            startDate=self.start_date,
            maxConcurrency=self.max_concurrency,
            toleratedFailurePercentage=self.tolerated_failure_percentage,
            toleratedFailureCount=self.tolerated_failure_count,
            itemCounts=self.item_counter.describe(),
            executionCounts=self.execution_counter.describe(),
        )
        stop_date = self.stop_date
        if stop_date is not None:
            describe_output["stopDate"] = self.stop_date
        return describe_output
    def list_item(self) -> MapRunListItem:
        list_item = MapRunListItem(
            executionArn=self.execution_arn,
            mapRunArn=self.map_run_arn,
            stateMachineArn=self.map_state_machine_arn,
            startDate=self.start_date,
        )
        if self.stop_date:
            list_item["stopDate"] = self.stop_date
        return list_item
    def update(